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The following explanations refer to sample questions from the "2007 USMLE Step 1: Content Description and Sample Test Materials", which is subject to the copyright policy of the National Board of Medical Examiners® (NBME). The explanations were submitted by WikiTestPrep users and are therefore subject to the GNU Free Documentation License.
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121. A
COPD describes patients with chronic bronchitis and/or emphysema who have a decreased forced expiratory volume. The overwhelming majority of patients with chronic bronchitis are smokers, up to 90% of all cases. Cigarette smoking also has been shown to be the major cause of emphysema. Therefore, decreasing the prevalence of cigarette smoking would lead to the greatest decrease in COPD in the community.
Persons with COPD are advised to get the flu and pneumococcal immunizations to help prevent serious infections in patients with compromised lung function. Patients with COPD have increased frequency and severity of lung infections and this likely plays a role in the etiology and progression of COPD, but not nearly to the extent of cigarette smoking. Therefore, increasing the rate of flu and pneumococcal immunizations would not lead to the greatest decrease of COPD.
Similarly, decreasing radon levels may decrease the prevalence of COPD, but only to a smaller extent when compared with cigarette smoking.
Increasing exercise will not decrease COPD in this community, especially if they continue to smoke cigarettes.
(94% of users answered this question correctly) Edit Explanation 122. C
A noncompetitive antagonist (C) binds at a different site than the receptor, or irreversibly binds the receptor site. In either case, the system is not activated. Thus, response is lowered at a given dose of agonist, and this effect is not overcome with increased agonist dose. (The dose-response curve shifts downward.) Drug W meets this description.
A competitive antagonist (A) reversibly binds to a receptor without activating the system, lowering the response at a given agonist dose. However, a response of maximum efficacy may be achieved with increased agonist dose. (The dose-response curve shifts to the right.) Drug W is not a competitive antagonist because it acts at a site other than the receptor for drug V. Further, it is not stated that the effect of drug W is overcome with increasing doses of drug V.
A full agonist (B) will produce a response of maximum efficacy if present in sufficient quantity. Drug W cannot be a full agonist, because it diminishes the physiologic effect of drug V.
When administered alone, a partial agonist (D) binds to the receptor site and elicits a less-than-100% response regardless of dose. When administered with a full agonist, it acts like an antagonist because it competes for receptor binding sites. (The dose-response curve shifts downward, and may also shift left or right.) Drug W cannot be a partial agonist because it acts at a site other than the receptor for drug V.
A reverse agonist (or inverse agonist) (E) binds to the same receptor site as an agonist, but elicits the opposite effect. This choice is incorrect because drug W does not bind at the same site as drug V, and only a diminished (rather than opposite) effect is observed.
(60% of users answered this question correctly) Edit Explanation 123. B
During the cephalic and gastric phases that occur after the child eats breakfast, secretions from the pancreas have high concentrations of digestive enzymes in the form of inactive precursors, including trypsinogen. The increased amount of trypsinogen in the duodenum is a result of stimulation of acinar cells by vagal input during cephalic phase and vago-vagal reflex secondary to stomach distention.
Degradation of trypsin does not lead to increased trypsinogen within the duodenum.
The rate of trypsinogen activation does not increase the amount of trypsinogen in the duodenum. It only increases the amount of activated trypsinogen or trypsin in the duodenum. Therefore, the actual amount of trypsinogen in the duodenum decreases as the trypsinogen is activated to trypsin.
Transcription of the trypsinogen gene and translation of the trypsinogen mRNA occur within the acinar cells of the exocrine pancreas and these processes increase the amount of trypsinogen within the acinar cells. However, it is the action of the vagus nerve to increase exocytosis of trypsinogen that leads to an increase of trypsinogen within the duodenum.
(75% of users answered this question correctly) Edit Explanation 124. C
Thyroid peroxidase is the enzyme responsible for iodide oxidation and binding to tyrosine in the follicle lumen.
The other steps in thyroid synthesis including cleavage of iodine from iodinated tyrosines, endocytosis of colloid, movement of iodide from plasma into thyroid cells, and separation of thyroxine from thyroglobulin in lysosomes, do not require thyroid peroxidase and would therefore not be inhibited in the transgenic animal.
(74% of users answered this question correctly) Edit Explanation 125. B
The woman experienced a panic attack, which can mimic heart attacks. After the resolution of a panic attack, the medical evaluation would be normal, which distinguishes these attacks from multiple heart attacks. The most appropriate nonpharmacologic therapy is cognitive behavioral therapy. The goal of this therapy is to change the way a patient interprets the world in order to change the patient's response to problems. For patients who experience panic attacks, cognitive behavioral therapy can help them to increase adaptive behavior and decrease emotional distress, thereby decreasing the amount of panic attacks.
Assertiveness training is a subtype of cognitive behavioral therapy. However, it is not the most appropriate therapy since it only address one aspect of the woman's disorder. Assertiveness training may help decrease social anxiety, but if that is not the only trigger for this woman's panic attacks, then it would not be sufficient for therapy.
Dynamic psychotherapy is a form of talking and relational therapy that is best used for adjustment, personality, depressive, and eating disorders. It also takes several months to years for the therapy to be effective. Therefore, it would not be appropriate for helping the woman with her recurrent panic attacks.
Psychoanalysis is a specific type of treatment where the analyst, upon hearing the thoughts of the patient, formulates and then explains the unconscious basis for the patient's symptoms and character problems. It can be used for panic attacks and anxiety disorders, but it would not be the most appropriate therapy since this woman would benefit more from relaxation training and systemic desensitization that are associated with cognitive behavioral therapy.
Psychodrama is a kind of group therapy in which group members act out their problems in a way to better understand conflicts between people. This type of therapy would not be appropriate for the woman and may even provoke another panic attack.
(68% of users answered this question correctly) Edit Explanation 126. E
The 9-month-old girl likely has phenylketonuria (PKU). In this disorder, there is a deficiency of phenylalanine hydroxylase or tetrahydrobiopterin cofactor, which causes a buildup of phenylalanine. State-mandated newborn screening looks for evidence of PKU to prevent the complications and developmental delays that occur in PKU when phenylalanine builds up in the body. The excess phenylalanine spills over into the urine as phenylacetic acid. The musty odor is a clue to the diagnosis. Since this is a disorder of aromatic amino acid metabolism, there is a musty odor in the diapers. Other common findings in PKU include fair skin, mental retardation, and growth retardation.
There would be an increased concentration of homocysteine in the urine in homocysteinuria, which is caused by cystathionine synthase deficiency, decreased affinity of cystathionine synthase for pyridoxal phosphate, or methionine synthase deficiency. Common findings with this disease include mental retardation, osteoporosis, tall statue, kyphosis, lens subluxation, and atherosclerosis.
Increased isoleucine in the urine likely result from blocked degradation of branched amino acids due to a lack of alpha-ketoacid dehydrogenase deficiency. This disease is maple syrup urine disease and the urine ofen smells like maple syrup.
Homogentisic acid in the urine is seen in alkaptonuria. In this disease, there is a deficiency of homogentisic acid oxidase. The common finding in this disease is that urine turns dark on standing and connective tissue may be darker in color.
Increased concentration of isovaleric acid in the urine can be found in isovaleric acidemia. Patients with isovaleric acidemia may present in early neonatal life with a sudden onset and severe illness. The illness usually begins within a few days of birth with increasing lethargy and decreased feeding, which often causes dehydration and weight loss. The classic odor in this disease is an "odor of sweaty feet" that represents the accumulation in the body of isovaleric acid and related compounds.
127. H
A 10-month-old child that can sit unassisted and has started to crawl has normal motor function. Infants do not start to walk until about 15 months. Her social and cognitive development are also normal. It is normal for infants to babble at 10 months and to be able to speak a few words by 15 months. Finally, the separation anxiety is also normal for infants between the ages of 10-15 months old.
If this 10-month-old girl was not able to sit unassisted, crawl, and babble, then her motor and cognitive development would be described as delayed because these are important milestones that occur in infants between 7-9 months. In addition, the presence of separation anxiety is normal for her age and up to 15-months-old and therefore is not a sign of delayed social development.
(79% of users answered this question correctly) Edit Explanation 128. A
Tinea corporis or ringworm is a fungal infection. An effective treatment for this infection is a topical azole that acts by blocking ergosterol synthesis and thereby increasing fungal membrane permeability. Miconazole and clotimazole are two azoles available in topical solutions that can be used to treat this infection.
Inhibition of DNA synthesis is the mechanism of action of flucytosine, which converts to fluorouracil and competes with uracil. This drug can be used to treat systemic fungal infections, but the side effects of nausea, vomiting, diarrhea, and bone marrow suppression limit its use.
Inhibition of ribosomal protein synthesis is the mechanism of action of various antibacterial agents including aminoglycosides, tetracyclines, chloramphenicol, erythromycin, Lincomycin, and Clindamycin. These drugs are used to treat various bacterial infections, but they are not used to treat fungal infections.
Interference with mycolic acid synthesis is how Isoniazid works. Isoniazid is a drug used for tuberculosis prophylaxis and in combination with other drugs for therapy for active tuberculosis. Isoniazid is not used to treat fungal infections.
Rifampin is another drug used in combination with Isoniazid for active tuberculosis therapy. It works by irreversibly binding to and inhibition of DNA-dependent RNA polymerase. Rifampin also is used for meningococcal prophylaxis, but it is not used to treat fungal infections.
(84% of users answered this question correctly) Edit Explanation 129. E
The midsystolic murmur heard best in the left third intercostal space associated with a thrill is consistent with the murmur of a ventricular septal defect (VSD). Of the answer choices above, only Tetralogy of Fallot includes a VSD. In addition to a VSD, patients with Tetralogy of Fallot also have pulmonary stenosis, right ventricular hypertrophy, and overriding aorta. These patients often present shortly after birth with cyanosis.
An atrial septal defect would present with a loud S1 and a wide, fixed split S2 on physical exam and it is unlikely that the patient would present with cyanosis.
The bicuspid aortic valve would not present with cyanosis. Many patients live with unrecognized bicuspid aortic valves until later in life with the development of aortic stenosis secondary to calcifications of the valve leaflets.
Patent ductus arteriosus likely would present with a continuous machine-like murmur.
Coarctation of the aorta is associated with elevated blood pressure in the upper extremities and decreased blood pressure in the lower extremities. If these patients present shortly after birth, they often have cyanosis of the lower extremity only.
(63% of users answered this question correctly) Edit Explanation 130. D
Visual accommodation occurs by contraction of the ciliary muscle to increase the curvature of the lens. Parasympathetic nerve fibers are responsible for ciliary muscle contraction and these act on muscarinic receptors. The muscarinic receptors are activated by acetylcholine to stimulate contraction of the ciliary body. Many antidepressant drugs have anti-cholinergic properties, thus blocking the muscarinic receptors and resulting in the loss of visual accommodation.
Antidepressant medication also may have anti-histaminergic and anti-serotoninergic properties. However, these two receptors are not found in the ciliary muscle and are not responsible for stimulating visual accommodation.
There are no beta-adrenergic receptors that stimulate the ciliary muscle of the eye. Therefore, blockade of beta-adrenergic receptors would not cause loss of visual accommodation.
There are alpha-adrenergic receptors located in the radial muscle of the eye, but this muscle is not responsible for visual accommodation.
(78% of users answered this question correctly) Edit Explanation 131. C
Hepatotoxicity is a well known side effect of isoniazid. Pyridoxine is given with isoniazid to help prevent B6 deficiency and neurotoxicity. The proximity of this man's symptoms to starting isoniazid, normal chest x-ray, and lack of exposure make tuberculosis and hepatitis B unlikely. Cholecystitis is not a well-known toxicity of pyridoxine and jaundice is not a typical feature of cholecystitis.
(94% of users answered this question correctly) Edit Explanation 132. B
Colchicine is effective in acute gout. It inhibits polymerization of microtubules and leukocyte chemotaxis.
(A) Allopurinol, (D) probenecid, and (E) sulfinpyrazone should be avoided during an acute flair because rapid changes in serum uric acid concentrations can precipitate crystalization of uric acid. Allopurinol inhibits uric acid production by inhibiting xanthene oxidase and probenecid inhibits uric acid reabsorption in the kidney. Sulfinpyrazone's mechanism of action is similar to probenecid.
(C) Morphine is a nonspecific treatment choice for gout. Colchicine is a better first choice.
133. E
Activated plasminogen (plasmin) degrades fibrin to fibrin split products.
(A) Heparin activates antithrombin III, which decreases the activity of IIa and Xa.
(B) Aspirin inhibits cyclooxygenase and thereby prostaglandin production by platelets.
(C) Factor II (prothrombin) is the inactive form of IIa (thrombin).
(D) TPA action is not dependent on fibrinogen concentration.
(72% of users answered this question correctly) Edit Explanation 134. D
Figure D shows a plateau of blood flow as mean arterial pressure (MAP) increases. This illustrates the phenomenon of autoregulation, in which tissues alter pre- or post-capillary resistance to keep blood flow relatively constant in the face of changing MAP. (The curve appears similar to those seen in pH buffer systems.)
Figure A shows a direct relationship between blood flow and MAP. This is an absence of autoregulation.
Figure B shows an inverse relationship between blood flow and MAP. This is abnormal and is not an example of autoregulation.
Figure C shows an increase in blood flow at a constant MAP. This is abnormal and is not an example of autoregulation.
Figure E shows a plateau of blood flow as MAP increases, but high blood flow should be seen with high MAP, not low.
(66% of users answered this question correctly) Edit Explanation 135. C
Eukaryotic genes contain introns and exons. Exons contain the genetic information that is transcribed into RNA while introns are spliced out during processing. Prokaryotic genes do not contain introns.
Both prokaryotic and eukaryotic cells contain (A) cell membranes, (B) DNA, (D) mRNA, and (E) ribosomes.
(72% of users answered this question correctly) Edit Explanation 136. E
This man likely had ventricular fibrillation secondary to ischemic heart disease given his age, history of angina and type 2 diabetes, and his sudden collapse and death. Ventricular fibrillation is the most common cause of sudden cardiac death in adults.
(A) Cardiac tamponade, (C) necrosis of the myocardium, and (D) rupture of the papillary muscle would be uncommon causes of sudden cardiac death and would most likely each present with their associated symptoms.
(B) Embolus to the right middle cerebral artery would produce a stroke and not sudden cardiac death.
(75% of users answered this question correctly) Edit Explanation 137. E
This woman's clinical and radiographic history is consistent with the diagnosis of sarcoidosis. Sarcoidosis is characterized by widespread noncaseating granulomas, elevated serum ACE levels, and is associated with, among other things, restrictive lung disease.
(A) Pulmonary hemorrhage is unlikely. There is no history of hemoptysis or evidence of hemorrhage on chest x-ray.
(B) Liquefactive necrosis is often associated with cellular destruction, often secondary acute infection.
(C) There is no suggestion of bronchiectasis or dilation of the respiratory bronchioles on history or radiography.
(D) Bacterial infection with abscess formation is unlikely given this woman's clinical history and radiographic findings.
(82% of users answered this question correctly) Edit Explanation 138. B
C5a is the major factor responsible for neutrophil chemotaxis. Activation of both the classical and alternative complement pathways results in C5a production and eventual membrane attack complex (MAC) formation.
(76% of users answered this question correctly) Edit Explanation 139. B
(B) In the adrenal gland, the zona glomerulosa produces mineralocorticoids, the zona fasciculata produces glucocorticoids, and the zona reticularis produces androgens. This patient's pituitary tumor was likely causing compression/destruction of ACTH-secreting cells as well as LH/FSH-secreting cells of the pituitary. This caused atrophy of the zona fasciculata and zona reticularis, respectively. The zona glomerulosa, which responds primarily to angiotensin II and serum potassium levels was spared. An acute stressor, such as this woman's surgery, will increase the body's need for cortisol and can lead to acute adrenal crisis in a case such as this.
We are not given any information that would lead us to believe there was autoimmune destruction, denervation, or granulomatous disease. This woman's cortisol deficiency was likely a cause of the tumor in her pituitary rather than a primary adrenal deficiency.
(77% of users answered this question correctly) Edit Explanation 140. D
(D) The most likely answer is irritable bowel syndrome. Alternating periods of constipation/diarrhea associated with psychosocial stress is typical of IBS. Physical and laboratory studies are normal in IBS.
(A) Gastroenteritis generally follows more of an acute timeline, does not wax and wane, constipation is not typically a component of the illness, and gastroenteritis implies that there may be nausea/vomiting from stomach involvement.
(B) Generalized anxiety disorder involves chronic anxiety often with little to provoke the anxiety and may include physical symptoms, but alternating periods of diarrhea/constipation is not a common complaint. Additionally, patients with GAD will tend to complain of anxiety related to a variety of everyday problems. This woman's anxiety is related specifically to her GI complaint.
(C) We are not given any information to lead us to a diagnosis of hypochondriasis. She has no history of multiple visits to the doctor or hospital with undiagnosed problems.
(E) We are not given information that would lead us to a diagnosis of major depressive disorder. This woman's anxiety is related to her GI complaints.
(92% of users answered this question correctly) Edit Explanation 141. D
Typically, body NaCl level is constant: NaCl excretion equals NaCl intake. However, administration of mineralocorticoid will cause the body to reabsorb more NaCl.
For each day, the difference between NaCl intake and NaCl excretion equals NaCl retention. On day 1 he retains 170 mmol NaCl; on day 2 he retains an additional 110 mmol NaCl; on day 3 he retains an additional 20 mmol NaCl. On day 4, NaCl excretion equals NaCl intake, showing he has reached a new steady-state. Thus, overall he has retained 170 + 110 + 20 = 300 mmol NaCl.
With free access to water and usual caloric intake, the retained NaCl is equivalent to addition of isotonic fluid. Given that 150 mmol NaCl weighs 1 kg, 300 mmol NaCl therefore weighs 2 kg. This 70-kg man will weigh 72 kg (answer D) at the end of day 4.
(59% of users answered this question correctly) Edit Explanation 142. A
Sildenafil (Viagra) is a commonly-used medication for erectile dysfunction and increasingly pulmonary hypertension. The drug is a potent cGMP phosphodiesterase 5 (PDE5) inhibitor. Thus, it blocks PDE5, which increases cellular cGMP in the corpus cavernosum. The increased cGMP promotes smooth muscle relaxation and vasodilitation, facilitating an erection.
A major side effect of sildenafil is that the above cascade can also occur in the general circulation at the level of the arteriole, thus causing systemic vasodilitation. This may result in dizziness and orthostatic hypotension. Such symptoms are especially worsened by nitrates such as nitroglycerin, which is contraindicated in patients taking sildenafil or other PDE5 inhibitors.
(76% of users answered this question correctly) Edit Explanation 143. E
Skin cancer comes in three major forms - squamous cell carcinoma (SCC), basal cell carcinoma (BCC), and melanoma. This patient's presentation and histology suggest squamous cell carcinoma. In general, a lesion described as ulcerated or with heaped borders is cancerous until proven otherwise. The atypical and dysplastic keratinocytes indicate a squamous cell origin.
Actinic keratosis is a thick, scaly lesion that a predecessor lesion to SCC. It would be fair to consider in this patient except that the clinical presentation describes a lesion that has progressed much further.
Melanoma is a neoplasm of melanocytes. Melanoma should be associated with ABCDE - Asymmetry, irregular Borders, multiple Colors, Diameter greater than 5mm, and Evolution of a pre-existing nevus. Melanoma is more common in younger people.
Mycosis fungoides is a presenation of cutaneous T-cell lymphoma; this patient's histology is not suggestive of such a rare diagnosis.
(72% of users answered this question correctly) Edit Explanation 144. A
Paternalism is when a physician places his/her values system as the primary values system to make decisions for the care of the patient instead of deferring to the patient's values or autonomous decisions. In this case, the wishes of the patient should be represented by the family members as there is no living will or advanced directives.
Preserving fairness would involve ethical decisions about who should receive scarce resources; a classic example deals with who should have priority in receiving organ transplants.
A good example of protecting patient autonomy is the process of informed consent before procedures or experiments.
Rationing care acknowledges scarce health resources must be appropriated based on their cost-effectiveness, even though an individual patient's health options may be diminished.
A good example of truth-telling would involve the disclosure to a patient of his/her prognosis.
(68% of users answered this question correctly) Edit Explanation 145. D
The test is attempting to measure the alveolar ventilation, or the amount of gas reaching the alveoli that is capable of gas exchange. The 20 micrometer particles are not arriving as they are too large to pass through the 10+ levels of conducting airways.
One can reach this answer through elimination of the others:
A - macrophage ingestion takes a long time, and would not affect a quick scan such as this. There is no evidence that nonuniform ingestion, if it exists, can explain the problem with this aerosol.
B - If the particles adhered to the walls, the more distal parts of the lung would be less ventilated by the particles, which would exacerbate the problem with the aerosol.
C - This would happen uniformly throughout the lung, which would not affect the measurement of alveolar ventilation.
E - The mucociliary system is exceedingly slow - it is an "elevator." This alone cannot explain the difficulty with the aerosol.
(73% of users answered this question correctly) Edit Explanation 146. C
The boy's wound appears to be superficial - he "scraped" his foot which is now infected (likely by a Gram positive organism such as Strep A or Staph). The superficial lymphatic drainage of the lower extremity roughly follows the superficial vessels. In this case of the lateral foot, the lymphatics drain along the path of the small saphenous vein to the popliteal fossa.
(66% of users answered this question correctly) Edit Explanation 147. E
Vitamin C deficiency (scurvy) classically presents with ecchymoses, petechaie, and gingival swelling. Vitamin C is necessary for the production of collagen; its deficiency results in a tendency towards weakened tissues and bleeding.
Beriberi is due to a deficiency of thiamine (B1). It may present as either "wet" (high-output heart failure and peripheral edema) or "dry" (mostly peripheral neuritis and paralysis). Either can also present with Wernicke's encephalopathy (ataxia, confusion, opthalmoplegia/nystagmus).
Kwashiorkor is found in children, generally under age 4, who suffer from severe protein malnutrition. Symptoms include edema, red hair, muscle wasting, and eventual death.
Pellagra is due to niacin (B3) deficiency and classically presents with the D's - dermatitis, dementia, and diarrhea.
Rickets is due to a vitamin D deficiency. It classically presents with symptoms/signs of osteomalacia or "softening" of the long bones - bowed legs, "knocked knees," greenstick fractures, costochondral pain, and especially growth disorders.
(90% of users answered this question correctly) Edit Explanation 148. E
Logically, a vaccine needs to work by inducing memory cells. There are two types of memory cells - T and B. Memory B cells (a variant of plasma cells) can only be produced after signaling from activated T cells. Thus, the vaccine will need to lead to the activation of T cells.
The carrier protein in the vaccine enables it to be taken up and processed by antigen-presenting cells (dendritic cells, B cells, macrophages). The peptides are presented on MHC type II, which contact and activate T-lymphocytes, starting the above immune cascade.
(71% of users answered this question correctly) Edit Explanation 149. D
The medial longitudinal fasciculus (MLF) is a necessary brainstem pathway for conjugate gaze. The MLF connects the third, fourth and sixth brainstem nuclei (in conjuction with vestibular input) to enable the coordination of eye movements in conjugate gaze. A lesion in the MLF will prevent conjugate gaze, but since the ocular muscles are still intact, convergence (controlled by a different neural pathway) is preserved. A lesion of the MLF is a common initial presentation of multiple sclerosis, and will present with the above scenario with diplopia.
Lesions of the lateral rectus muscle, medial rectus muscle, or nucleus of the oculumotor nerve would affect both conjugate gaze and convergence.
A lateral rectus palsy will lead to difficulty abducting the ipsilateral eye. A medial rectus palsy will lead to difficulty adducting the ipsilateral eye. A oculumotor nerve lesion could present with ptosis and an eye that is abducted and vertically deviated (owing to an overpowering fourth and sixth cranial nerve).
(74% of users answered this question correctly) Edit Explanation 150. E
Norepinephrine and dopamine have been implicated in cycles of addiction. In the CNS, dopamine is involved with learning and limbic systems; addiction may viewed as a maladjustment of these neural pathways. Glutamate has recently been considered to play a role in cocaine addiction specifically. The role of norepinephrine in addiction is more variable.
In the CNS, there are a few key clinical correlates of neurotransmitters to remember:
Acetylcholine deprivation is involved in Alzheimer's dementia
Serotonin is involved in mood - SSRI's and SNRI's work to increase serotonin at the postsynaptic junction. Serotonin has also been implicated in addiction mechanisms.
GABAergic channels are mediated by benzodiazepenes, barbituates, and sedatives.
(64% of users answered this question correctly) Edit Explanation |