Edward F. Hollinger, M.D., Ph.D., Troy Pittman, M.D.
1 A 41-year-old woman undergoes complex repair of a deep laceration in her hand. When removing the dressing on postoperative day 2, a large clot with mild surrounding erythema is encountered. Which of the following statements regarding the inflammatory phase of wound healing is true?
A It lasts up to 24 hours after the injury is incurred.
B Initial vasodilation is followed by subsequent vasoconstriction.
C Bradykinin causes vasoconstriction, which inhibits migration of neutrophils to the healing wound.
D The complement component C5a and platelet factor attract neutrophils to the wound.
E The presence of neutrophils in the wound is essential for normal wound healing.
Ref.: 1-5
Comments
The inflammatory phase starts immediately after the injury occurs and lasts up to 72 hours. After the injury, there is a transient period (about 10 minutes) of vasoconstriction followed by active vasodilation. These events are mediated by substances released secondary to the local tissue injury. Vasoactive components such as histamine cause brief periods of vasodilation and increased vascular permeability. The kinins (bradykinin and kallidin) are released by the enzymatic action of kallikrein, which is formed after activation of the coagulation cascade. These components, in addition to those of the complement system, stimulate the release of prostaglandins (particularly PGE1 and PGE2), which work in concert to maintain more prolonged vessel permeability, not only of capillaries but also of larger vessels. In addition, these substances, particularly the complement component C5a and platelet-derived factors such as platelet-derived growth factor (PDGF), act as chemotactic stimuli for neutrophils to enter the wound. Although neutrophils can phagocytize bacteria from a wound, the results of studies involving clean wound healing show that healing can proceed normally without them. Monocytes, however, must be present for normal wound healing because in addition to their role in phagocytosis, they are required to trigger a normal fibroblast response. The later phases of wound healing include the proliferative or regenerative phase and the remodeling phase. The proliferative phase is marked by the appearance of fibroblasts in the wound, which leads to the formation of granulation tissue. The remodeling phaseinvolves an increase in wound strength secondary to collagen remodeling and lasts up to 1 year after the initial injury. The three main phases of wound healing may occur sequentially or simultaneously.
Answer
D
2 A 55-year-old woman with a history of venous stasis ulcers is evaluated for a nonhealing ulcer on the medial aspect of the lower part of her leg. Application of topical ointment to the ulcer and compression stockings have allowed partial healing. However, she states that regardless of the various interventions, the ulcer never completely heals. Which of the following statements regarding wound epithelialization is true?
A Integrins act as a key modulator of the interaction between epithelial cells and the surrounding environment.
B Structural support and attachment between the epidermis and dermis are provided by tight cell junctions.
C Early tensile strength of the wound is a direct result of collagen deposition.
D A reepithelialized wound develops hair follicles and sweat glands like those seen in normal skin.
E Contact inhibition can prevent collagen deposition and result in a chronic (nonhealing) wound.
Ref.: 2, 4-6
Comments
Migration of epithelial cells is one of the earliest events in wound healing. Shortly after injury and during the inflammatory phase, basal epithelial cells begin to multiply and migrate across the defect, with fibrin strands being used as the support structure. Integrins are the main cellular receptors involved in epithelial migration; they act as sensors and integrators between the extracellular matrix and the epithelial cell cytoskeleton. Tight junctions within the epithelium contribute to its impermeability, whereas the basement membrane contributes to structural support and attachment of the epidermis to the dermis. Surgical incisions seal rather promptly and after 24 hours are protected from the external environment. Early tensile strength is a result of blood vessel ingrowth, epithelialization, and protein aggregation. After covering the wound, the epithelial cells keratinize. The reepithelialized wound has no sweat glands or hair follicles, which distinguishes it from normal skin. Control of the cellular process during wound epithelialization is not completely understood, but it appears to be regulated in part by contact inhibition, with growth being arrested when two or more similar cells come into surface contact. Derangements in the control of this process can result in epidermoid malignancy. Malignancy is more frequently observed in wounds resulting from ionizing radiation or chemical injury, but it can occur in any wound when the healing process has been chronically disrupted. For example, squamous cell carcinoma may develop in patients with chronic burn wounds or osteomyelitis (Marjolin ulcer).
Answer
A
3 A 31-year-old man undergoes his second exploratory laparotomy for bowel obstruction secondary to Crohn’s disease. The patient expresses concern regarding the long-term complications related to his midline incision since he has taken steroids for the last year. Which of the following statements regarding the role of collagen in wound healing is true?
A Collagen synthesis in the initial phase of injury is the sole responsibility of endothelial cells.
B Net collagen content increases for up to 2 years after injury.
C At 3 weeks after injury, more than 50% of the tensile strength of the wound has been restored.
D Tensile strength of the wound increases gradually for up to 2 years after injury; however, it generally reaches a level of only about 80% of that of uninjured tissue.
E Tensile strength is the force necessary to reopen a wound.
Ref.: 2, 3, 6
Comments
Synthesis of collagen by fibroblasts begins as early as 10 hours after injury and increases rapidly; it peaks by day 6 or 7 and then continues more slowly until day 42. Collagen continues to mature and remodel for years. Its solubility in saline solution and the thermal shrinkage temperature of collagen reflect the intermolecular cross-links, which are directly proportional to collagen age. After 6 weeks, there is no measurable increase in net collagen content. However, synthesis and turnover are ongoing for life. Historical accounts of sailors with scurvy (with impaired collagen production) who experienced reopening of previously healed wounds illustrate this fact. Tensile strength correlates with total collagen content for approximately the first 3 weeks of wound healing. At 3 weeks, the tensile strength of skin is 30% of normal. After this time, there is a much slower increase in the content of collagen until it plateaus at about 6 weeks. Nevertheless, tensile strength continues to increase as a result of intermolecular bonding of collagen and changes in the physical arrangement of collagen fibers. Although the most rapid increase in tensile strength occurs during the first 6 weeks of healing, there is slow gain for at least 2 years. Its ultimate strength, however, never equals that of unwounded tissue, with a level of just 80% of original skin strength being reached. Tensile strength is measured as the load capacity per unit area. It may be differentiated from burst strength, which is the force required to break a wound (independent of its area). For example, in wounds of the face and back, burst strength is different because of differences in skin thickness, even though tensile strength may be similar. Corticosteroids affect wound healing by inhibiting fibroblast proliferation and epithelialization. The latter effect can be reversed by the administration of vitamin A.
Answer
D
4 Which of the following is correct regarding cell signaling?
A Cytokines are exclusively peptide mediators.
B Autocrine mediators are secreted by a cell and act on adjacent cells of a different type.
C Cytokines are usually produced by cells specialized for only that purpose.
D The effects of hormones are generally local rather than global.
E Growth factors are frequently mediated by second messenger systems such as diacylglycerol (DAG) and cyclic adenosine monophosphate (cAMP).
Ref.: 7-9
Comments
Cytokines are proteins, glycoproteins, or peptides that bind to target cell surface receptors to stimulate a cellular response. They are important mediators of wound healing. Cytokines can reach target cells by paracrine, autocrine, or intracrine routes. Paracrine mediators are produced by one cell and act on an adjacent target cell. Autocrine mediators are secreted by a cell and act on cell surface receptors on the same cell. Intracrine mediators act within a single cell. Hormones are released by cells and act on a distant target (endocrine route). Although the distinction between cytokines and hormones has blurred, in general, hormones are secreted from specialized glands (e.g., insulin, parathyroid hormone), and cytokines are secreted by a wide variety of cell types. Hormones typically induce body-wide effects, whereas the effects of cytokines may be more localized (e.g., wound healing at the site of an injury). Generally, growth factors are named according to their tissue of origin or their originally discovered action. Growth factors interact with specific membrane receptors to initiate a series of events that ultimately lead to stimulation of cell growth, proliferation, or differentiation. The intermediate events activate a variety of second messenger systems mediated by agents such as inositol 1,4,5-triphosphate (IP3), DAG, and cAMP.
Answer
E
5 A 25-year-old man is seen in the office with complaints of contracture of his left index finger after a burn injury. Which of the following statements is true about growth factors?
A Epidermal growth factor (EGF) stimulates the production of collagen.
B Vascular endothelial growth factor (VEGF) and PDGF both stimulate angiogenesis by binding to a common receptor.
C Fibroblast growth factor (FGF) stimulates wound contraction.
D Transforming growth factor-β (TGF-β) is stored in endothelial cells.
E Tumor necrosis factor-α (TNF-α) inhibits angiogenesis.
Ref.: 3, 6, 10, 11
Comments
Epidermal growth factor was the first cytokine described. It is a potent mitogen for epithelial cells, endothelial cells, and fibroblasts. EGF stimulates synthesis of fibronectin, angiogenesis, and collagenase activity. Platelet-derived growth factor is released from the alpha granules of platelets and is responsible for the stimulation of neutrophils and macrophages and for increasing production of TGF-β. PDGF is a mitogen and chemotactic agent for fibroblasts and smooth muscle cells and stimulates angiogenesis, collagen synthesis, and collagenase activity. Vascular endothelial growth factor is similar to PDGF but does not bind to the same receptors. VEGF is mitogenic for endothelial cells. Its role in promoting angiogenesis has led to interest in anti-VEGF therapies for cancer. Fibroblast growth factor has acidic and basic forms whose actions are identical but whose strengths differ (basic FGF is 10 times stronger than acidic FGF). FGF is mitogenic for endothelial cells, fibroblasts, keratinocytes, and myoblasts; stimulates wound contraction and epithelialization; and induces the production of collagen, fibronectin, and proteoglycans. It is an important mediator of angiogenesis. Transforming growth factor-β is released from the alpha granules of platelets and has been shown to regulate its own production in an autocrine manner. TGF-β stimulates fibroblast proliferation and the production of proteoglycans, collagen, and fibrin. It is an important mediator of fibrosis. Administration of TGF-β has been suggested as an approach to reduce scarring and reverse the inhibition of wound healing by glucocorticoids. Tumor necrosis factor-α is a mitogen for fibroblasts and is produced by macrophages. It stimulates angiogenesis and the synthesis of collagen and collagenase.
Answer
C
6 A 34-year-old man sustained a gunshot wound to his abdomen that necessitated exploratory laparotomy and small bowel resection. Two weeks after the initial operation, he was reexplored for a large intraabdominal abscess. Which of the following will result in the most rapid gain in strength of the new incision?
A A separate transverse incision is made.
B The midline scar is excised with a 1-cm margin.
C The midline incision is reopened without excision of the scar.
D The midline incision is left to heal by secondary intention.
E The rate of gain in strength is not affected by the incision technique.
Ref.: 2, 3, 6
Comments
When a normally-healing wound is disrupted after approximately the fifth day and then reclosed, return of wound strength is more rapid than with primary healing. This is termed the secondary healing effectand appears to be caused by elimination of the lag phase present in normal primary healing. If the skin edges more than about 7 mm around the initial wound are excised, the resulting incision is through essentially uninjured tissue, so accelerated secondary healing does not occur.
Answer
C
7 A 29-year-old black woman is scheduled for incision and drainage of a breast abscess that has recurred three times despite ultrasound-guided needle drainage. The patient has a history of keloid formation and is concerned about an unsightly scar on her breast. Which of the following statements concerning wound healing is true?
A Keloids contain an overabundance of fibroblasts.
B A hypertrophic scar extends beyond the boundaries of the original wound.
C Improvement is usually seen with keloid excision followed by intralesional steroid injection.
D An incision placed perpendicular to the lines of natural skin tension will result in the least obvious scar.
E Hypertrophic scars occur most commonly on the lower extremities.
Ref.: 2, 3, 6
Comments
Keloids are caused by an imbalance between collagen production and degradation. The result is a scar that extends beyond the boundaries of the original wound. The absolute number of fibroblasts is not increased. Treatment of keloids is difficult. There is often some improvement with excision and intralesional steroid injection. If this technique is not successful, excision and radiation treatment can be used. Hypertrophic scars contain an overabundance of collagen, but the dimensions of the scar are confined to the boundaries of the original wound. Hypertrophic scars are often seen in the upper part of the torso and across flexor surfaces. Scar formation is affected by multiple factors, including the patient’s genetic makeup, wound location, age, nutritional status, infection, tension, and surgical technique. In planning surgical incisions, an effort to parallel natural tension lines will promote improved wound healing.
Answer
C
8 An 85-year-old nursing home patient is found to have a worsening stage III sacral pressure ulcer. The ulcer is débrided and tissue for culture obtained. Tissue cultures reveal 108 organisms per gram of tissue after operative débridement. What is the next most appropriate step in management of the patient’s wound?
A Muscle flap coverage
B Wound vacuum-assisted closure (VAC)
C Intravenous antibiotics
D Repeat débridement
E Débridement with immediate application of a split-thickness skin graft
Ref.: 2, 3, 6, 12
Comments
The National Pressure Ulcer Advisory Panel has recommended a staging system for pressure sores that is useful in planning treatment. Stage I is represented by the presence of nonblanching erythema of intact skin. Stage II is characterized by partial-thickness skin loss involving the epidermis or dermis. Clinically, the ulcer is manifested as a blister, abrasion, or a shallow crater. Stage III is full-thickness skin loss with involvement of the underlying subcutaneous tissue. Stage III wounds may extend down to but not through the underlying fascia. Stage IV represents full-thickness skin loss with extensive destruction or tissue necrosis of underlying structures, which may include muscle and bone. Studies have shown that wounds with quantitative cultures revealing more than 106 organisms per gram of tissue that undergo reconstruction with skin or even muscle flaps have a significantly greater risk for complications, including infection, accumulation of fluid, and wound dehiscence. Similarly, a skin graft is unlikely to survive in an environment with such a high bacterial inoculate. Negative pressure wound therapy, such as with the wound vacuum-assisted closure system, involves the use of a sponge and an occlusive dressing connected to a suction apparatus in a closed system. In patients with large wounds, a wound VAC may serve as a bridge to reduce wound size for definitive reconstruction. It has been shown to be effective in reducing wound edema, controlling wound drainage, encouraging diminution of wound size, and facilitating the formation of granulation tissue. Although studies show that wound VAC therapy may reduce bacterial counts over time, the most appropriate management of this patient is repeat débridement of the wound. Intravenous antibiotics may be indicated to treat underlying osteomyelitis.
Answer
D
9 A 30-year-old man is scheduled for definitive management of his open wounds after undergoing embolectomy and fasciotomies on his left lower extremity. Which of the following statements is true regarding the use of split- and full-thickness skin grafts?
A A split-thickness skin graft undergoes approximately 40% shrinkage of its surface area immediately after harvesting.
B A full-thickness skin graft undergoes approximately 10% shrinkage of its surface area immediately after harvesting.
C Secondary contraction is more likely to occur after adequate healing of a full-thickness skin graft than after adequate healing of a split-thickness skin graft.
D Sensation usually returns to areas that have undergone skin grafting.
E Skin grafts may be exposed to moderate amounts of sunlight without changing pigmentation.
Ref.: 2, 3, 6
Comments
Skin grafts are considered to be full thickness when they are harvested at the dermal-subcutaneous junction. Split-thickness skin grafts are those that contain epidermis and variable partial thicknesses of underlying dermis. They are usually 0.018 to 0.060 inch in thickness. Cells from epidermal appendages deep to the plane of graft harvest resurface the donor site of a split-thickness skin graft in approximately 1 to 3 weeks, depending on the depth. The donor site requires a moist environment to promote epithelialization, and such an environment is maintained by using polyurethane or hydrocolloid dressings. Because a full-thickness graft removes all epidermal appendages, the defects must be closed primarily. When a skin graft is harvested, there is immediate shrinkage of the surface area of the graft. This process, known as primary contraction, is due to recoil of the elastic fibers of the dermis. The thicker the skin graft, the greater the immediate shrinkage, with full-thickness grafts shrinking by approximately 40% of their initial surface area and split-thickness grafts shrinking by approximately 10% of their initial surface area. Shrinkage must be considered when planning the amount of skin to harvest for covering a given size wound. Secondary contraction occurs when contractile myofibroblasts in the bed of a granulating wound interact with collagen fibers to cause a decrease in the wound’s surface area. Secondary contraction is greater in wounds covered with split-thickness grafts than in those covered with full-thickness grafts. The amount of secondary contracture is inversely proportional to the amount of dermis included in the graft rather than the absolute thickness of the graft. Dermal elements hasten the displacement of myofibroblasts from the wound bed.
Sensation may return to areas that have been grafted as long as the bed is suitable and not significantly scarred. Although sensation is not completely normal, it is usually adequate for protection. This process begins at about 10 weeks and is maximal at 2 years. Skin grafts appear to be more sensitive than normal surrounding skin to melanocyte stimulation during exposure to ultraviolet sunlight. Early exposure to sunlight after grafting may lead to permanently increased pigmentation of the graft and should be avoided. Dermabrasion or the application of hydroquinones may be of benefit in reducing this pigmentation.
Answer
D
10 A 45-year-old woman undergoes bilateral transverse rectus abdominis muscle (TRAM) breast reconstruction after modified radical mastectomy. The patient is scheduled for postoperative radiation therapy and is concerned that this will affect her ability to heal her wounds. Which of the following statements regarding wound healing in this patient is true?
A Denervation has a profound effect on wound contraction and epithelialization.
B A bacterial count of 1000 organisms per square centimeter retards wound healing.
C Chemotherapy beginning 10 to 14 days after primary wound closure has little effect on the final status of a wound.
D Tissue ischemia is the main component of tissue damage after irradiation.
E Postoperative radiation therapy should be delayed at least 4 to 6 months after surgery to decrease the incidence of wound complications.
Ref.: 2-4, 6, 13
Comments
Denervation has no effect on wound contraction or epithelialization. Flap wounds in paraplegics heal satisfactorily when other factors, such as nutrition and temperature, are controlled. Subinfectious bacterial levels appear to accelerate wound healing and the formation of granulation tissue. However, when the level reaches 106 organisms per square centimeter of wound, healing is delayed because of decreased tissue oxygen pressure, increased collagenolysis, and a prolonged inflammatory phase. Various chemotherapeutic agents affect wound healing. Most antimetabolic agents (e.g., 5-fluorouracil) do not delay wound healing, although agents such as doxorubicin have been shown to delay wound healing. When chemotherapy begins 10 to 14 days after wound closure, little effect is noted on its final status despite a demonstrable early retardation in wound strength. Tissue ischemia may not be the primary factor involved in chronic wound-healing problems associated with irradiation. Such problems are most likely related to changes within the nuclei and concomitant cytoplasmic malformation. To decrease wound complications, it is usual to delay surgery until at least 3 to 4 weeks after full-dose irradiation and to avoid radiation therapy for at least 3 to 4 weeks after surgery.
Answer
C
11 A 21-year-old graduate student has a large hypertrophic scar on the lower part of her face. The patient had sustained a laceration on her face 2 years previously after hitting her face on the side of a swimming pool. Which of the following statements regarding scar revision is true?
A Scar maturation refers to the change in size of the wound in the first 1 to 2 months.
B Scar revision should have been performed in the first 3 months after injury to minimize fibrosis.
C Revision should be performed earlier in children than in adults.
D It corrects undesirable pigmentation.
E Scar revision should be delayed approximately 1 year to allow maturation.
Ref.: 2, 3, 6
Comments
Changes in pliability, pigmentation, and configuration of a scar are known as scar maturation. This process continues for many months after an incision, so it is generally recommended that revision not be carried out for approximately 12 to 18 months because natural improvement can be anticipated within this period. In general, scar maturation occurs more rapidly in adults than in children. Most erythematous scars show little improvement after revision, therefore scar revision should not be undertaken for correction of undesirable scar color alone.
Answer
E
12 A 68-year-old diabetic man undergoes a below-knee amputation. The patient’s postoperative course is complicated by severe depression and anorexia. Before discharge the patient is started on a multivitamin regimen. Which of the following statements regarding wound healing is true?
A Vitamin A is needed for hydroxylation of lysine and proline in collagen synthesis.
B High doses of vitamin C improve wound healing.
C Vitamin E is involved in the stimulation of fibroplasia, collagen cross-linking, and epithelialization.
D Zinc deficiency results in delayed early wound healing.
E Iron deficiency had been linked to defects in long-term wound remodeling.
Ref.: 2, 3, 6
Comments
Vitamin A is involved in the stimulation of fibroplasia and epithelialization. Although there has been no conclusive evidence of efficacy in humans, in animal studies vitamin A has been shown to reverse the inhibitory effects of glucocorticoids on the inflammatory phase of wound healing and epithelialization. Vitamin C is a necessary cofactor in the hydroxylation and cross-linking of lysine and proline in collagen synthesis. Deficiencies in vitamin C (scurvy) can lead to the production of inadequately hydroxylated collagen, which either degrades rapidly or never forms proper cross-links. Doses higher than physiologic doses do not improve wound healing. Vitamin E is applied to wounds and incisions by many patients, but there is no evidence to support the use of vitamin E in wound healing. Large doses of vitamin E have been found to inhibit wound healing. Zinc is a necessary cofactor of RNA and DNA polymerase, and deficiencies have been linked to poor early wound healing. Iron (specifically, the ferrous iron) is necessary for converting hydroxyproline to proline. However, chronic anemia and iron deficiency have not been linked to delayed or impaired wound healing.
Answer
D
13 A 46-year-old man is evaluated shortly after undergoing radiation therapy and chemotherapy for primary laryngeal cancer. He also gives a history of long-term steroid use for rheumatoid arthritis. The patient complains of a chronic, nonhealing wound on his neck, just over his right clavicular head. Which statement regarding the treatment of this wound is true?
A The wound should be treated with compression dressings.
B The wound should be treated with injected steroids.
C The patient should start taking vitamin A, and the wound should be covered with antimicrobial dressings.
D The patient should start taking vitamin C, and the wound should be kept open to air.
E The wound should be excised and a skin graft applied.
Ref.: 11, 14
Comments
Radiation results in progressive endarteritis obliterans and microvascular damage to the skin, which leads to skin ischemia and fibrotic interstitial changes. This leaves wounds in the skin particularly prone to infection. The use of antimicrobial dressings capable of maintaining a moist environment is ideal for these wounds. Research also supports the use of hyperbaric oxygen and growth factors to promote wound healing. Patients taking steroids should receive daily vitamin A supplements. Wounds in these patients show decreased rates of angiogenesis, collagen deposition, and cellular proliferation. Wounds should be kept free of bacterial contamination.
Answer
C
14 A 25-year-old ballet dancer with a history of anorexia nervosa arrives at the emergency department with right lower quadrant pain. After an appendectomy, a wound infection at the surgical site requires débridement. The patient is placed on an antibiotic regimen, and the wound is packed with wet-to-dry dressings. Regarding wound healing and malnutrition, which of the following statements is true?
A Hypoproteinemia leads to decreased levels of arginine and glutamine, which are essential in wound healing.
B Cell membranes rapidly become dehydrated in the absence of vitamin E, resulting in delayed wound healing.
C Zinc is essential to the fibroblast’s ability to cross-link collagen.
D Vitamin D serves an immunomodulatory role in wound healing.
E The patient should be treated with high-dose vitamin C, vitamin A, and zinc.
Ref.: 2, 15
Comments
Adequate amounts of protein, carbohydrates, fatty acids, and vitamins are essential for wound healing. Hypoproteinemia results in decreased delivery of the essential amino acids used in the synthesis of collagen. Carbohydratesand fats provide energy for wound healing, and in their absence, proteins are rapidly broken down. Fatty acids are vital components of cell membranes. Vitamin C is a cofactor for hydroxylation of lysine and proline during collagen synthesis, and deficiency leads to decreased collagen cross-linking by fibroblasts. Vitamin C is also effective in providing resistance to infection. Vitamin Ais essential for normal epithelialization, proteoglycan synthesis, and enhanced immune function. Vitamin D is required for normal calcium metabolism, but it is also involved in promoting immune function in the skin. Vitamin E has not been shown to play a role in wound healing. Zinc deficiency leads to deficient formation of granulation tissue and inhibition of cellular proliferation. Increased administration of vitamins and minerals does not accelerate wound healing and often has a deleterious effect.
Answer
D
15 A 56-year-old man underwent total thyroidectomy for papillary cancer. On the first postoperative day, the patient complains of circumoral tingling and muscle weakness. Which of the following statements regarding the electrical properties of cell membranes is not true?
A Ions flow through hydrophilic channels formed by specific transmembrane proteins.
B Lipids provide the ability to store electric charge (capacitance).
C Active pumps maintain the ionic gradients necessary for a resting membrane potential.
D Initiation of an action potential depends on voltage-gated channels.
E Large numbers of sodium ions rush in during the initial phase of a nerve action potential.
Ref.: 16, 17
Comments
This patient has clinical findings associated with hypocalcemia. Specific transmembrane proteins provide hydrophilic paths for the ions (primarily Na+, K+, Ca2+, and Cl−) involved in electrical signaling. The amino acid sequence in specific regions of these proteins determines the selectivity for ions. The lipid component of the plasma membrane provides the capability of storing electric charge (capacitance), and the protein component provides the capability of resisting electric charge (resistance). Establishment and maintenance of a resting cell membrane potential requires the separation of charge maintained by membrane capacitance, selective permeability of the plasma membrane, concentration gradients (intracellular versus extracellular) of the permeant ions, and impermeant intracellular anions. Active pumping by the sodium (sodium-potassium adenosine triphosphatase [Na+,K+-ATPase]) or calcium pumps generally maintains the ionic concentration gradients. Action potentials are regenerative (self-sustaining) transient depolarizations caused by the activation of voltage-sensitive sodium and potassium channels. Only a small volume of sodium ions is necessary to initiate an action potential. In fact, the amount of sodium ions that flow into a typical nerve cell during an action potential would change the intracellular Na+ concentration by only a few parts per million.
Answer
E
16 An 84-year-old woman with colon cancer undergoes a right hemicolectomy. Her estimated blood loss is 700 mL. Shortly after surgery, her urine output falls to 10 mL/h. She is administered several liters of normal saline. On the second postoperative day, the patient complains of severe swelling of her hands and feet. Which cell junction acts as a transmembrane linkage without an intracellular communication function?
A Tight junction
B Gap junction
C Desmosome
D Connexon
E All of these junctions have an intracellular communication function
Ref.: 16, 18
Comments
Any patient undergoing abdominal surgery will sustain a certain amount of capillary leakage. A proposed mechanism involves increased release of nitric oxide, which causes vasodilation in precapillary cells, vasoconstriction in postcapillary cells, and ultimately results in increased third-spacing of fluids. There are three major types of cell junctions: gap junctions, desmosomes, and tight junctions. Gap junctionsare the most common and function primarily in intercellular communication but also in cellular adhesion. The connection between cells maintained by a gap junction is not particularly stable; it depends on a variety of complexes on each cell but not on connecting proteins (hence the term gap). Gap junctions serve as a pathway of permeability between cells for many different molecules up to weights of 1000 daltons. Connexons are protein assemblies formed by six identical protein subunits. They span the intercellular gap of the lipid bilayer to form an aqueous channel connecting the bilayers. Desmosomesfunction as cellular adhesion points but do not provide a pathway of communication. They are linked by filaments that function as transmembrane linkers, but desmosomes are not points of true cell fusion. Tight junctions, in contrast, are true points of cell fusion and are impermeable barriers. They prevent leakage of molecules across the epithelium in either direction. They also limit the movement of membrane proteins within the lipid bilayer of the plasma membrane and therefore maintain cells in a differentiated polar state.
Answer
C
17 A 42-year-old woman with a history of end-stage renal disease is being evaluated for cadaveric renal transplantation. Which of the following statements regarding cell surface antigens is true?
A Cell surface antigens are generally glycoproteins or glycolipids.
B Histocompatibility antigens are not cell surface antigens.
C ABO antigens are glycoproteins.
D ABO antibodies are present at birth.
E HLA antigens have an extracellular hydrophobic region and an intracellular hydrophilic region.
Ref.: 19
Comments
Cell surface antigens are generally glycoproteins or glycolipids that are anchored to either a protein or a lipid. Common examples include the ABO blood group antigens and the histocompatibility antigens. Antigens of the ABO system are glycolipids whose oligosaccharide portions are responsible for the antigenic properties. The structures of the blood group oligosaccharides occur commonly in nature and lead to the stimulation needed to produce anti-A or anti-B antibodies after a few months of life. HLA antigens are two-chain glycoproteins that are anchored in the cell membrane at the carboxyl terminal. These antigens contain an extracellular hydrophilic region, a transmembrane hydrophobic region, and an intracellular hydrophilic region. This transmembrane structure allows extracellular signals to be transmitted to the interior of the cell.
Answer
A
18 A 36-year-old man is evaluated at the office because of complaints of fatigue, weight gain, and irritability. Routine laboratory tests are performed and his thyroid-stimulating hormone (TSH) level is found to be 11.0. The patient is concerned about his condition and inquires about the relationship between hypothyroidism and his symptoms. Which of the following statements regarding second messenger systems is true?
A Most receptor proteins (such as G proteins) are completely extracellular.
B Both the “first messenger” and “second messenger” mediators of cell signaling function within the cell cytoplasm.
C Adenylate cyclase stimulates the conversion of cAMP to adenosine triphosphate (ATP).
D IP3 generally increases cytoplasmic calcium concentrations.
E IP3 and DAG together lead to inactivation of protein kinase C.
Ref.: 7, 8, 16
Comments
The thyrotropin (TSH) receptor is a Gαs receptor found mainly on the surface of thyroid follicular cells. When activated, it stimulates increased production of thyroxine (T4) and triiodothyronine (T3).
Several families of receptor proteins have been identified. The most common is the G protein (guanine nucleotide–binding protein) family, a subset of guanosine triphosphatase (GTPase) enzymes. All G protein–coupled receptors have a characteristic seven transmembrane domains. Binding of an extracellular ligand causes a conformational change in the receptor that allows it to exchange guanosine diphosphate (GDP) for guanosine triphosphate (GTP) on the intracellular portion of the G protein. The intracellular portion of the “large” (heterotrimeric) G protein–coupled receptor consists of three subunits, Gα, Gβ, and Gγ. Other “small” (monomeric) G protein receptors have only a homologue of the Gα portion. There are several important subsets of the “large” G protein receptors, and they are classified according to the specific intracellular pathway that is activated.
Gαs stimulates membrane-associated adenylate cyclase to produce cyclic adenosine monophosphate from ATP. cAMP is a second messenger that activates protein kinase A, which results in the phosphorylation of downstream targets. Gαs ligands include adrenocorticotropic hormone (ACTH), calcitonin, glucagon, histamine (H2), TSH, and many others. Gαi inhibits the production of cAMP from ATP. Gαi ligands include acetylcholine (M2 and M4), dopamine (D2, D3, and D4), and histamine (H3 and H4). Gαq activates phospholipase C, which cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-triphosphate and diacylglycerol. IP3 mediates the release of calcium from intracellular reservoirs, such as the endoplasmic reticulum (ER), sarcoplasmic reticulum (SR) in muscle, and mitochondria. IP3 and DAG together work to activate protein kinase C, which can modulate membrane permeability and activate gene transcription. Gαq ligands include histamine (H1), serotonin (5-HT2), and muscarinic receptors.
The most well known “small” G protein receptors are the Ras family GTPases. The Ras receptors influence a wide variety of processes in the cell, including growth, cellular differentiation, and cell movement.
Chemical messengers can influence intracellular physiology via several mechanisms. Some ligands, such as acetylcholine (binding to the nicotinic cholinergic receptor) or norepinephrine (binding to the potassium channel in cardiac muscle), directly bind to ion channels in the cell membrane to alter their conductance. Some lipid-soluble messengers, such as steroid and thyroid hormones, enter the cell and bind to nuclear or cytoplasmic receptors, which then bind to DNA to increase transcription of selected mRNA. Many other extracellular messengers bind to the extracellular portion of transmembrane receptor proteins to trigger the release of intracellular mediators. The extracellular ligands are termed the “first messenger,” whereas the intracellular mediators are “second messengers.” Examples of second messengers include IP3, DAG, calcium, and cAMP.
Answer
D
19 A 67-year-old man undergoes revascularization of his right lower extremity after sustaining thrombosis secondary to a popliteal artery aneurysm. Shortly after surgery, a compartment syndrome of the affected limb develops and is attributed to reperfusion injury. Research suggests that ER stress may be responsible for apoptosis after ischemia. Which of the following statements regarding the ER is not true?
A Rough ER is a primary site of lipid synthesis.
B Smooth ER plays an important role in the metabolism of drugs.
C Ribosomes attached to the rough ER manufacture proteins for use within the cell.
D The SR is found mainly in epithelial cells.
E The SR plays an important role in gluconeogenesis.
Ref.: 18, 20
Comments
The endoplasmic reticulum is part of a network that includes mitochondria, lysosomes, microbodies, the Golgi complex, and the nuclear envelope. This network forms an intracellular circulatory system that allows vital substrates to reach the interior of the cell for transportation and assembly. There are two types of ER. Rough endoplasmic reticulum is coated with ribosomes and functions as the site of synthesis of membrane and secreted proteins. Other ribosomes that circulate freely in the cytoplasm synthesize proteins destined to remain within the cell. Smooth endoplasmic reticulum plays a major role in metabolic processes, including the synthesis of lipids and steroids, metabolism of carbohydrates (especially gluconeogenesis), drug detoxification, and molecular conjugation. Smooth ER contains the enzyme glucose-6-phosphatase, which converts glucose-6-phosphate to glucose during gluconeogenesis. Cells that synthesize large amounts of protein for export have abundant rough ER, whereas cells that make steroids (e.g., those in the adrenal cortex) generally have smoother ER. The smooth ER is continuous with the nuclear envelope. The sarcoplasmic reticulum is a distinct type of smooth ER found in striated and smooth muscle. The SR contains large stores of calcium, which it sequesters and then releases when the cell is stimulated. Release of calcium from the SR plays a major role in excitation-contraction coupling, which allows muscle cells to convert an electric stimulus to a mechanical response.
Answer
B
20 A 43-year-old woman is undergoing external beam radiation therapy for invasive breast cancer. Biopsy of the tumor shows a relatively high mitotic index, indicative of active growth. Which portion of the cell cycle in actively dividing cells is most sensitive to ionizing radiation?
A S phase
B M phase
C G1 phase
D G2 phase
E All phases are equally radiosensitive
Ref.: 21
Comments
The primary mechanism by which ionizing radiation induces cell death is direct and indirect injury to deoxyribonucleic acid (DNA). Ionizing radiation can cause lethal damage (damage that cannot be repaired; for example, most double-strand DNA breaks) or sublethal damage (damage that can be repaired if conditions are correct; for example, most single-strand DNA breaks). Factors that increase the cell’s ability to repair damage make it less sensitive to ionizing radiation. The cell division cycle is divided into four distinct phases. Replication of DNA occurs in the synthesis (S) phase, whereas nuclear division and cell fission occur in the mitotic (M) phase. The intervals between these two phases are called the gap (G) phases. Cells in M phase (mitosis) have the least capability to repair sublethal damage and hence are the most radiation sensitive. Cells in S phase have the most capability of repairing damage and consequently are the most radiation resistant. Resting cells (G0) are less sensitive to radiation injury than cells that are actively dividing (and proceeding through the cell cycle). Cancer cells are generally less differentiated (with less ability to repair DNA damage) and more rapidly dividing than normal tissue. The fact that tumor cells are usually more sensitive to radiation than surrounding normal tissue is an important determinant of the utility of radiation therapy.
Answer
B
21 A 56-year-old man is transferred from the county jail with complaints of hemoptysis, fever, and chills. The patient had undergone left lower lobectomy 6 years ago for an isolated lung nodule. Chest radiography on admission shows a lesion in the left upper lobe that is concerning for tuberculosis. The cell wall of Mycobacterium tuberculosis prevents lysosomes from fusing with phagosomes, which contributes to its tendency to lead to granuloma formation. Which of the following statements regarding endocytosis is not true?
A Phagocytosis refers to engulfment of particulate matter.
B Pinocytosis refers to the engulfment of soluble material.
C Only specialized cells of the immune system are capable of endocytosis.
D Opsonins increase the likelihood of phagocytosis by binding to the antigen.
E Antibodies and complement fragments can serve as opsonins.
Ref.: 7, 20
Comments
All cells are capable of endocytosis, which is the process of internalizing extracellular molecules by engulfing the molecule within the cell membrane. Pinocytosis (cell drinking) is the engulfment of soluble material. Phagocytosis(cell eating) is the process by which cells ingest solids. For cells of the immune system, such as macrophages, dendritic cells, and polymorphonuclear leukocytes, phagocytosis is particularly important in recognizing and combating pathogens. In phagocytosis the cell membrane surrounding the engulfed material pinches off and forms a vesicle called a phagosome. The phagosome maintains the material separate from the cytosol of the cell. The phagosome fuses with a lysosome, which leads to degradation of the engulfed material. Degradation can be oxygen dependent (by the production of reactive oxygen species) or okygen independent (generally by proteolytic enzymes and cationic proteins).
Typically, both the target (antigen) and the phagocyte are negatively charged. This limits their ability to come into close proximity. Opsonins are molecules that act to enhance phagocytosis. Opsonizationoccurs when antigens are bound by antibody or complement molecules (or both). Phagocytic cells express receptors (Fc, CR1) that bind opsonin molecules (antibody, C3b), which greatly increases the affinity of the phagocyte for the antigen. Phagocytosis is an unlikely event if the antigen is not opsonized.
Answer
C
22 Which of the following statements regarding lysosomes is true?
A Primary lysosomes usually contain extracellular material targeted for digestion.
B Lysosomal enzymes work effectively in the acidic pH of the cytoplasm.
C Serum levels of lysosomal acid phosphatases may have prognostic value in diseases such as prostate cancer.
D Lysosomal storage diseases such as Tay-Sachs result from unregulated activity of lysosomal enzymes.
E To better isolate their hydrolytic enzymes, lysosomes are resistant to fusion with other cell membranes.
Ref.: 18, 20
Comments
Lysosomes are membrane-bound organelles that contain acid hydrolases. Heterolysosomes are involved in the endocytosis and digestion of extracellular material, whereas autolysosomes are involved in digestion of the cell’s own intracellular material. Primary lysosomes are formed by the addition of hydrolytic enzymes (from the rough ER) to endosomes from the Golgi complex. Combining a primary lysosome with a phagosome creates a phagolysosome. Lysosomal enzymes are hydrolases that are resistant to autolysis. They function best in the acidic milieu of the lysosome; the slightly alkaline pH of the surrounding cytosol helps protect the cell from injury if the lysosome leaks. Acid phosphatase is a marker enzyme for lysosomes. Different forms of acid phosphatase are found in lysosomes from various organs, and serum levels may be indicative of disease (for example, prostatic acid phosphatase may have prognostic significance in prostate cancer).
One of the distinguishing characteristics of lysosomal membranes is their ability to fuse with other cell membranes. Lysosomal membranes have a high proportion of lipids in a micellar configuration, primarily because of the presence of the phospholipid lysolecithin. This increased micellar configuration facilitates fusion of the lysosome membrane with the phagosome membrane for digestion and with the plasma membrane for secretion. Steroids are thought to work partially by stabilizing lysosomal membranes, thereby inhibiting membrane fusion and enzyme release. Lysosomes may engage in autophagocytosis, which is thought to be important for cell turnover, cell remodeling, and tissue changes. Several lysosomal storage diseases, such as Tay-Sachs, Gaucher, and Pompe disease, are caused by inactive or missing lysosomal digestive proteins. These genetic diseases lead to the accumulation of normally degraded substrates within the cell.
Answer
C
23 An 81-year-old woman undergoes a Hartman procedure for perforated diverticulitis. Postoperatively, the patient remains hypotensive and norepinephrine is administered. On day 2, parenteral nutrition is initiated. Which of the following statements regarding oxidative phosphorylation and mitochondria is true?
A Glycoproteins are transported into the mitochondrial matrix to facilitate oxidative phosphorylation.
B The citric acid cycle takes place within the inner mitochondrial membrane.
C Oxidative phosphorylation via ATP synthase converts adenosine diphosphate (ADP) to ATP.
D Electrochemical (proton) gradients provide the energy to power chemosmotic production of ATP.
E Mitochondrial DNA is almost exclusively paternally derived.
Ref.: 22
Comments
Metabolic substrates such as fats, proteins, and glycoproteins are converted to fatty acids and pyruvate and transported into mitochondria. Within the mitochondrial matrix they are metabolized by the citric acid (Krebs) cycle to produce the reduced forms of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2). The reducing power of these substrates fuels transfer of electrons from electron donors to receptors as oxidative phosphorylation. The resultant high-energy electrons pass along the electron transport chain and release the energy used to move protons across the inner mitochondrial membrane to generate potential energy in the form of electrical and pH gradients. ATP synthase uses the energy obtained from allowing protons to flow down this gradient to synthesize adenosine triphosphate from adenosine diphosphate. This process is called chemosmosis. Three ATP molecules are generated for each mole of oxygen consumed. Mitochondrial DNA is transmitted only from the mother because sperm contains few mitochondria. During sepsis, inhibited mitochondrial function as a result of hypoxia or other mediators of sepsis has been postulated to contribute to organ injury through accelerated oxidant production and by promoting cell death.
Answer
D
24 Inflammatory breast cancer is diagnosed in a 36-year-old woman. A decision is made to treat the patient with radiation, along with paclitaxel and doxorubicin. Which of the following statements regarding cellular motility and contractility is true?
A Actin fibers are found mainly in muscle cells.
B The interactions between actin and myosin that underlie the contraction of skeletal muscle require calcium but not ATP.
C Intermediate filaments extend from the centrosome to the nucleus.
D The proteins kinesin and dynein are required for directional transport of cellular components along the microtubules.
E The microtubules used to form the spindle apparatus are synthesized de novo before each mitosis.
Ref.: 8, 16
Comments
The cytoskeleton provides the structural framework for the cell. It is composed of three main types of protein polymers: actin filaments, intermediate filaments, and microtubules. Actin filaments are found in nearly all types of cells. They form a cortical layer beneath the plasma membrane of most cells, the stress fibers of fibroblasts, and the cytoskeleton of microvilli of intestinal epithelial cells. In muscle cells, the interaction between the heads of myosin (thick filaments) and actin (thin filaments) requires hydrolysis of ATP to separate the filaments at the end of the power stroke. Calcium and troponin C (an actin-associated protein) are also required to expose the binding site for myosin on the actin filament. Intermediate filaments are a heterogeneous group of proteins that extend from the nucleus to the cell surface. They interact with other cytoskeletal filaments and binding proteins to produce their effects.
Microtubules arise from the centrosome, with the cell’s microtubule-organizing center being located near the nucleus. Microtubules are in a constant dynamic equilibrium between assembly and disassembly. Movement of cellular components, such as vacuoles, along the microtubules requires ATP and either of two associated proteins: kinesin for movement away from the centrosome and dynein for movement toward it. Cilia and flagella contain columns of doublet microtubules in a 9-2 arrangement (nine doublets in a circle surrounding two central doublets). Movement is accomplished when the doublets slide along each other in a process mediated by dynein and fueled by hydrolysis of ATP. Microtubules also play an important role in cell division. Assembly of the mitotic spindle involves replication and splitting of the microtubule-organizing center into the two spindle poles and reorganization of the cytoskeletal microtubules to form the spindle apparatus. Taxanes function as mitotic inhibitors by inhibiting depolymerization of the mitotic spindle, which results in a “frozen” mitosis. Paclitaxel is a natural taxane that prevents depolymerization of cellular microtubules. The vinca alkaloids (e.g., vinblastine, vincristine) also inhibit cell division, but by disrupting the mitotic spindle. Doxorubicin (Adriamycin) intercalates between DNA base pairs and impairs the progression of topoisomerase II, which unwinds DNA for transcription.
Answer
D
25 A 26-year-old with a history of type 2 neurofibromatosis is scheduled to undergo resection of an acoustic neuroma. The NF2 gene is located on the long arm of chromosome 22. Which of the following statements regarding chromosomes is not true?
A The nucleus contains the entire cellular DNA.
B Histones compact and organize the DNA strands.
C Interactions between DNA and proteins expose specific genes and control their expression.
D During mitosis, the spindle apparatus attaches to the chromosome at the centromere.
E Telomeres maintain chromosomal length through the replication cycles.
Ref.: 23
Comments
Chromosomes are formed by the combination of double-stranded helical DNA with histones and other proteins. The interactions between DNA and proteins stabilize the chromosomal structure. Most cellular DNA is located in the nucleus, although a small portion is found in the mitochondria. Each chromosomal double helix contains approximately 108 base pairs. There are several levels of organizational restructuring, from DNA and histones binding to form chromatin all the way to the complex folded structure of the chromosome itself. To express a gene, that portion of the chromosome must be unfolded and unwrapped to expose the DNA double helix. Gene expression is regulated by the binding of nonchromosomal proteins, called transcription factors, to specific regions of the DNA (enhancer and promoter sequences). Several distinct regions of chromosomes are identifiable: the origins of replication (sites of initiation of DNA synthesis), the centromere (site of spindle attachment during mitosis), and telomeres (specialized end structures that maintain the length of the chromosome through replication cycles).
Answer
A
26 A 25-year-old man is admitted to the trauma intensive care unit after sustaining multiple gunshot wounds to the abdomen that necessitated several small bowel resections. On postoperative day 12 the patient begins to have spiking fevers. Blood cultures grow Serratia, and the patient is started on an antibiotic regimen that includes gentamicin. Aminoglycosides bind the ribosomal 30S subunit, thereby inhibiting bacterial protein production. Which of the following statements regarding protein synthesis is not true?
A Transcription of messenger RNA occurs in the nucleus.
B Messenger RNA moves from the nucleus to the cytoplasm and attaches to free ribosomes in the cytoplasm.
C The enzyme RNA polymerase catalyzes the transcription of messenger RNA from DNA.
D Introns are placed into the DNA transcript by splicing.
E Posttranslational processing includes glycosylation and enzymatic cleavage.
Ref.: 24
Comments
The sequence of nucleotides in DNA determines the amino acid sequence of the protein. Protein synthesis involves (1) transcription of messenger RNA from the gene that codes for the protein, (2) translation of the messenger RNA into a protein, and (3) posttranslational processing of the protein, which may involve enzymatic cleavage or glycosylation of the protein. Transcription takes place in the nucleus, whereas translation and posttranslational processing occur in the rough ER, Golgi complex, or free ribosomes in the cytoplasm. Transcription of messenger RNA from DNA occurs by assembly of complementary base pairs on the DNA template one nucleotide at a time. This step is catalyzed by the enzyme RNA polymerase. Eukaryotic genes are interrupted by noncoding regions called introns. Introns are removed from the RNA transcript by splicing. The resulting messenger RNA is moved to the cytoplasm, in which it binds to ribosomes to begin translation. The initial step in protein synthesis is attachment of the messenger RNA to a ribosome that is preloaded with transfer RNA that recognizes the start codon (three bases) AUG and thus sets the reading frame for the translation. Subsequent binding of aminoacyl-transfer RNA to the ribosomes that match the three nucleotide codons specifying each amino acid results in peptide synthesis as the ribosome moves along the messenger RNA molecule. The first portion of the protein that is synthesized is an amino terminal leader called the signal peptide. At this stage, the ribosome becomes attached to the rough ER. As translation continues, the signal peptide is inserted into the rough ER membrane by another transmembrane protein and later cleaved as the peptide elongates.
Answer
D
27 A 27-year-old woman sustains an incomplete T10 spinal cord injury after falling off a horse. The patient is given 30 mg/kg of methylprednisolone. Which of the following is true regarding steroid hormones and their receptors?
A Steroid hormones are synthesized from proteins.
B In the bloodstream, steroid hormones often dimerize to facilitate transport.
C Steroid hormone receptors are found only in the cytoplasm.
D Heat shock proteins (HSPs) are usually associated with cytosolic steroid hormone receptors.
E Binding of the steroid hormone to a receptor induces a second messenger cascade to alter cellular metabolism.
Ref.: 16
Comments
Steroid hormones are synthesized from cholesterol. Their lipophilic nature allows them to cross cell membranes easily. Steroid hormones can be divided into five groups based on their receptors: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestogens. In the bloodstream, steroid hormones are generally bound to specific carrier proteins such as sex hormone–binding globulin or corticosteroid-binding globulin. Receptors for steroid hormones are most commonly located in the cytosol, although they are also found in the nucleus and on the cell membrane. After binding to the steroid hormone, steroid receptors often dimerize. For many cytosolic steroid receptors, binding of the ligand induces a conformational change and releases heat shock proteins. Nuclear steroid receptors are not generally associated with HSPs. HSPs themselves have several roles, including functioning as intracellular chaperones for other proteins, serving as transcription factors, and facilitating antigen binding. They may also serve as targets for therapeutics. Ultimately, the activated steroid receptor must enter the nucleus to serve as a transcription factor for augmentation or suppression of the expression of particular genes. The resulting messenger RNA leaves the nucleus for the ribosomes, where it is translated to produce specific proteins.
Answer
D
28 A 55-year-old man with a history of hepatitis C cirrhosis has complaints of nausea, fever, and progressive lethargy. Part of his evaluation includes an assessment of his hepatitis C viral load. Which of the following tests would be most useful in assessing his hepatitis C viral load?
A Western blot
B Gel electrophoresis
C Fluorescence microscopy
D Polymerase chain reaction (PCR)
E Expression cloning
Ref.: 16
Comments
Western blot is a technique used to detect specific proteins in a sample. An antibody to the protein of interest is used as a probe. Gel electrophoresis is a method for separating proteins or nucleic acids according to size, mass, or composition. It is based on the differential rate of movement of the molecules of interest through a gel when an electric field is applied. Polymerase chain reaction is a technique by which DNA may be massively amplified. Primers or oligonucleotides are synthesized to complement one strand of the DNA to be amplified. Amplification involves three temperature-cycled steps: (1) heating for separation (denaturation) of the double-helix structure into two single strands, (2) cooling for hybridization of each single strand with its primer (annealing), and (3) heating for DNA synthesis (elongation). The steps are repeated with exponential amplification of the DNA of interest. When RNA is used, reverse transcriptase is employed initially to transcribe the RNA to DNA before amplification. Quantitative polymerase chain reaction can be used in real time to measure the starting concentration of DNA or RNA in a sample, for example, the amount of hepatitis C RNA in a blood sample. With expression cloning, DNA coding for a protein of interest is cloned into a plasmid (extrachromosomal DNA molecule) that can be inserted into a bacterial or animal cell. The cell expresses the protein, which allows the production of sufficient amounts for study. Fluorescence microscopy is performed by labeling a component of interest in a sample with a molecule that absorbs light at one wavelength and emits light at another (fluorescence).
Answer
D
29 Which of the following methods is most useful for determining the RNA content of a sample?
A Southern blotting
B Northern blotting
C Western blotting
D PCR
E None of the above
Ref.: 24
Comments
Blotting is a method used to study macromolecules (DNA, RNA, or proteins) separated by gel electrophoresis (usually by size) and transferred onto a carrier (technically, the transfer is the “blot”). The macromolecules can then be visualized by specific probes or staining methods. A Southern blot is used for detection of specific DNA sequences. A Northern blot performs the same function but for RNA or mRNA samples. A Western blot is used to detect specific proteins in a sample, with an antibody to the protein of interest being used as a probe. An Eastern blot is a modification of the Western blot technique that is used to detect posttranslational modification of proteins. There are several other modifications of the technique; for example, Southwestern blotting is used to detect DNA-binding proteins. The origin of the nomenclature is derived from the Southern blot, which is named for its inventor biologist Edwin Southern.
Answer
B
30 What enzyme is responsible for the catalysis of deoxynucleoside triphosphates into DNA?
A DNA helicase
B DNA ligase
C DNA polymerase
D DNA primase
E All of the above
Ref.: 23
Comments
DNA polymerases are enzymes that catalyze the assembly of deoxynucleoside triphosphates into DNA. There are several types of DNA polymerases. DNA polymerase III promotes DNA elongation by nucleotide linkage, whereas DNA polymerase I functions to fill gaps and repair DNA. DNA helicase is the enzyme involved in unwinding the double-stranded DNA into individual strands before replication, transcription, or repair. DNA primase catalyzes the formation of RNA primers used to initiate DNA synthesis. DNA ligase joins the DNA fragments generated by the degradation of RNA primers.
Answer
C
31 In DNA replication, what type of mutation is specifically associated with the generation of a stop codon?
A Point mutation
B Missense mutation
C Nonsense mutation
D Frameshift mutation
E Neutral mutation
Ref.: 7
Comments
A change in a single base pair is known as a point mutation. A single amino acid change resulting from a point mutation is known as a missense mutation. A missense mutation may cause changes in the structure of the protein that lead to altered biologic activity. Nonsense mutations occur if a point mutation results in the replacement of an amino acid codon with a stop codon. Nonsense mutations lead to premature termination of translation and often result in the loss of encoded protein. Frameshift mutations occur when a few base pairs are added or deleted and lead to the introduction of unrelated amino acids or stop codons. A neutral mutation occurs when the change results in the substitution of a different but chemically similar amino acid. Frequently, the amino acids are similar enough that little or no change occurs in the resultant protein.
Answer
C
References
1 Alarcon LH, Fink MP. Mediators of the inflammatory response. In Townsend CM, Beauchamp RD, Evers BM, et al, editors: Sabiston textbook of surgery: the biological basis of modern surgical practice, ed 18, Philadelphia: WB Saunders, 2008.
2 Ethridge RT, Leong M, Phillips LG. Wound healing. In Townsend CM, Beauchamp RD, Evers BM, et al, editors: Sabiston textbook of surgery: the biological basis of modern surgical practice, ed 18, Philadelphia: WB Saunders, 2008.
3 Fine NA, Mustoe TA. Wound Healing. In Mulholland MW, Lillemoe KD, Doherty GM, et al, editors: Greenfield’s surgery: scientific principles and practice, ed 4, Philadelphia: Lippincott Williams & Wilkins, 2006.
4 Simmons RL, Steel DL. Basic science review for surgeons. Philadelphia: WB Saunders; 1992.
5 Gupta S, Lawrence WT. Wound healing normal and abnormal mechanisms and closure techniques. In O’Leary JP, Tabuenca A, editors: The physiologic basis of surgery, ed 4, Philadelphia: Lippincott Williams & Wilkins, 2008.
6 Barbul A, Efron DT. Wound healing. In Brunicardi FC, Andersen DK, Billiar TR, et al, editors: Schwartz’s principles of surgery, ed 9, New York: McGraw-Hill, 2010.
7 Ko TC, Evers BM. Molecular and cell biology. In Townsend CM, Beauchamp RD, Evers BM, et al, editors: Sabiston textbook of surgery: the biological basis of modern surgical practice, ed 18, Philadelphia: WB Saunders, 2008.
8 Williams JA, Dawson DC. Cell structure and function. In Mulholland MW, Lillemoe KD, Doherty GM, et al, editors: Greenfield’s surgery: scientific principles and practice, ed 4, Philadelphia: Lippincott Williams & Wilkins, 2006.
9 Rosengart MR, Billiar TR. Inflammation. In Mulholland MW, Lillemoe KD, Doherty GM, et al, editors: Greenfield’s surgery: scientific principles and practice, ed 4, Philadelphia: Lippincott Williams & Wilkins, 2006.
10 Peacock EE. Symposium on biological control of scar tissue. Plast reconstr surg. 1968;41:8-12.
11 Barbul A. Immune aspects of wound repair. Clin Plast Surg. 1990;17:433-442.
12 Galiano RD, Mustoe TA. Wound care. In Aston S, Seasley R, Thorne C, editors: Grabb and Smith’s plastic surgery, ed 6, Philadelphia: Lippincott-Raven, 2007.
13 Basson MD, Burney RE. Defective wound healing in patients with paraplegia and quadriplegia. Surg Gynecol Obstet. 1982;155:9-12.
14 Gurtner GC. Wound Healing: Normal and Abnormal. In Aston S, Seasley R, Thorne C, editors: Grabb and Smith’s plastic surgery, ed 6, Philadelphia: Lippincott-Raven, 2007.
15 Martindale RG, Zhou M. Nutrition and metabolism. In O’Leary JP, Tabuenca A, editors: The physiologic basis of surgery, ed 4, Philadelphia: Lippincott Williams & Wilkins, 2008.
16 Reeves ME. Cell biology. In O’Leary JP, Tabuenca A, editors: The physiologic basis of surgery, ed 4, Philadelphia: Lippincott Williams & Wilkins, 2008.
17 Transport across cell membranes. In: Lodish H, Berk A, Zipursky SL, et al, editors. Molecular cell biology. New York: Scientific American Books, 1999.
18 Biomembranes and the subcellular organization of eukaryotic cells. In: Lodish H, Berk A, Zipursky SL, et al, editors. Molecular cell biology. New York: Scientific American Books, 1999.
19 Protein sorting, organelle biogenesis and protein secretion. In: Lodish H, Berk A, Zipursky SL, et al, editors. Molecular cell biology. New York: Scientific American Books, 1999.
20 The dynamic cell. In: Lodish H, Berk A, Zipursky SL, et al, editors. Molecular cell biology. New York: Scientific American Books, 1999.
21 Radiosensitivity and cell age in the mitotic cycle. In Hall EJ, Amato JG, editors: Radiobiology for the radiologist, ed 6, Philadelphia: Lippincott Williams & Wilkins, 2005.
22 Cellular energetics, glycolysis, aerobic oxidation and photosynthesis. In: Lodish H, Berk A, Zipursky SL, et al, editors. Molecular cell biology. New York: Scientific American Books, 1999.
23 DNA replication, repair and recombination. In: Lodish H, Berk A, Zipursky SL, et al, editors. Molecular cell biology. New York: Scientific American Books, 1999.
24 Recombinant DNA and genomics. In: Lodish H, Berk A, Zipursky SL, et al, editors. Molecular cell biology. New York: Scientific American Books, 1999.