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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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31. D
Trimethoprim-sulfamethoxazole is a sulfa-based drug. Sulfonamides are known inhibitors of the P-450 enzymatic system, and warfarin is primarily metabolized by CYP450:2C9. Trimethoprim-sulfamethoxazole would be expected to increase the serum levels of warfarin and push the INR to super-therapeutic levels with the increased risk of bleeding. Therefore, it would be recommended to decrease the dosage of warfarin (choice D).
Begining therapy with vitamin K (choice A) is the treatment for warfarin overdose, which this patient does not have. Additionally, giving vitamin K will drop the INR to sub-therapeutic levels which will place this patient with a newly placed prosthetic valve at risk for thromboembolisms.
Increasing the dosage of warfarin (choice B) would not be recommended in this patient because trimethoprim-sulfamethoxazole will already increase the serum levels of warfarin. Further increasing the dosage of warfarin will definitely place this patient at risk for bleeding.
Making no alterations in the dosage of warfarin (choice C) is not recommended because of the risk of bleeding with a supra-therapeutic INR as a result from inhibited warfarin metabolism.
Stopping the warfarin and changing to low-dose aspirin (choice E) is not recommended because this patient with a newly placed prosthetic valve still requires anti-coagulation to prevent thromboembolisms. Additionally, aspirin has not been shown to be adequately effective in anti-coagulating patients with prosthetic valves.
(66% of users answered this question correctly) Edit Explanation 32. C
The deficient enzyme must be common to both pathways of converting glycogen to glucose and of converting fructose to glucose to yield the observed disease. Only Glucose-6-phosphatase (choice C) is common to both pathways. Glucose-6-phosphatase is required in the conversion of Glucose-6-phosphate to glucose, which is the common last step in the pathway of converting both glycogen and fructose to glucose. Deficiency of glucose-6-phosphatase is also known as Von Gierke’s disease or Type I glycogen storage disease.
Fructokinase (choice A) converts fructose to fructose-1-phosphate—the first step in the metabolism of fructose. Deficiency in this enzyme should not effect the conversion of glycogen to glucose. Fructokinase deficiency can lead to a benign disorder called essential fructosuria.
Glucokinase (choice B) is a liver-specific enzyme that converts glucose to glucose-6-phosphate as the first step of glucose metabolism. Deficiency in this enzyme should not effect conversion of glycogen to glucose or conversion of fructose to glucose.
Phosphoglucomutase (choice D) is an enzyme that allows inter-conversion between glucose-1-phosphate and glucose-6-phosphate. Deficiency in this enzyme will impair conversion of glycogen to glucose, but will not impair conversion of fructose to glucose.
UDPG-glycogen transglucosylase (choice E) is an enzyme that facilitates the incorporation of glucose into glycogen. Deficiency of this enzyme will impair glycogen synthesis.
(65% of users answered this question correctly) Edit Explanation 33. B
Mullerian-inhibitory stubstance (choice B) is a hormone secreted by the Sertoli cells of the testes during embryogenesis of the fetal male. It inhibits the development of the Mullerian ducts, which would develop into the upper vagina, uterus and cervix, and fallopian tubes, if uninhibited.
Estrogen (choice A and D) and testosterone (choice C and E) have no role in the development of the mullerian ducts.
(77% of users answered this question correctly) Edit Explanation 34. C
Often when patients refuse to comply with a physician's recommendations, there is a reason for the refusal. It is important to explore the patient's reasons for refusal. This woman has explained why she does not want a mammography. She is afraid of the discomfort involved. Therefore offering analgesia (choice C) is a reasonable first option to try to allay her fears, and have her agree to the procedure.
In general, fear tactics (choice A and D), and ultimatums (choice B and E) will not be the correct answer on the USMLE Step 1 exam.
(82% of users answered this question correctly) Edit Explanation 35. E
This man suffered from Korsakoff syndrome. His history of alcoholism predisposed him to a thiamine (B1) deficiency, a vitamin that is crucial to cellular energy production. A deficiency of this vitamin causes Wernicke-Korsakoff syndrome. Wernicke encephalopathy is characterized by confusion, ocular disturbance, and ataxia of gait. Korsakoff syndrome manifests as loss of short-term memory and confabulation. The lesion of both of these disorders is found in the mammillary bodies (choice E) of the thalamus.
The amygdala (choice A) is an almond-shaped nuclear mass in the medial part of the anterior temporal lobe. It is involved in processing memory and emotion.
The caudate nucleus (choice B) is a nucleus located in the basal ganglia involved in the control of voluntary movement.
The hippocampus (choice C) is a part of the limbic system found in the medial temporal lobe. It is involved in short term memory and and spatial navigation.
The locus coeruleus (choice D) is a nucleus located in the brain stem. It is involved in the stress response, and secretes norepinephrine.
36. C
Gastrin and histamine (choice C) both result in increased HCl secretion from parietal cells. Gastrin is produced by the G cells of the antrum and duodenum, and it stimulates parietal cells, as well as increases histamine release from enterochromaffin cells. Parietal cells also have receptors for histamine, which stimulate them to secrete gastric acid.
Acetylcholine will increase gastric acid secretion, but secretin (choice A and E) stimulates secretion of bicarbonate from the pancreas, neutralizing acid.
Cholecystokinin (choice B and E) stimulates gallbladder contraction and pancreatic enzyme secretion.
Vasoactive intestinal polypeptide (choice D) stimulates water and electrolyte secretion from the intestines via cAMP.
(83% of users answered this question correctly) Edit Explanation 37. E
Sensitivity, as defined in biostatistics, is the number of true positive results divided by the total positive test results (true positive plus false negative).
Sensitivity = 60/(60+20) = 0.75
38. D
Blood pressures in the pulmonary artery (choice D) are in the range of 15-30mmHg systolic and 4-12mmHg diastolic, which are pressures that most closely match those found in this patient's catheterization.
Left atrial (choice C) pressures range from 1-10mmHg, and right atrial (choice E) pressures range from 0-8mmHg. Prior to birth, the ductus arteriosus (choice A) diverts blood from the pulmonary artery to the aorta, and the foramen ovale (choice B) provides a communication between the two atria, equalizing their pressures, and shunting blood from the pulmonary circulation to the systemic circulation.
(69% of users answered this question correctly) Edit Explanation 39. B
This question involves the relation of predictive value to disease diagnosis. Choosing Point B as the cutoff will cause the test to have a relatively high false-positive rate: some people with concentrations within the normal range will test positive for the disease. However, because this disease is irreversible and fatal unless treated early, it is appropriate in this case to set a cutoff point that will not generate any false-negative results. Thus, setting the cutoff point at B will ensure that the negative predictive value of the test is 100%, at the expense of having a somewhat lower positive predictive value.
Choice A is incorrect because the false-positive rate would be so high that the test would not be clinically useful (the positive predictive value will be too low).
Choices C-E are incorrect because all these choices would give some false-negative results (the negative predictive value will not be 100%).
(70% of users answered this question correctly) Edit Explanation 40. B
This infant has the clinical presentation of cystic fibrosis, which is due to defective ion transport at epithelial surfaces (choice B). This is an autosomal recessive disease due to a mutation in the chloride transporter, cystic fibrosis transmembrane conductance regulator (CFTR). Patients present with meconium ileus, deficiencies of pancreatic enzymes, pulmonary obstruction, frequent pulmonary infection, bronchiectasis, cor pulmonale, and respiratory failure. Other findings include liver cirrhosis, infertility, and elevated NaCl concentrations in sweat.
This is not due to an autoimmune disorder (choice A), disaccharidase deficiency (choice C), inability to synthesize apolipoprotein B (choice D), or villous atrophy of the jejunum (choice E).
(77% of users answered this question correctly) Edit Explanation 41. B
Dextrocardia (choice B) is a congenital malformation of the heart in which the heart is situated on the right side of the body. These patients require ECG leads to be placed in the reverse position. The condition is usually asymptomatic.
Bifid heart (choice A) is a congenital heart defect where the heart is split, or separated into two parts.
Double-outlet right ventricle (choice C) is a congenital heart disease where both the pulmonary artery and the aorta arise from the right ventricle.
Persistent truncus arteriosus (choice D) is a cyanotic congenital heart disease where a single arterial vessel arises from the base of the heart, and splits in to the coronary, systemic, and pulmonary arteries.
Transposition of the great vessels (choice E) is also a cyanotic congenital heart disease, where the aorta arises from the right ventricle, and the pulmonary artery from the left ventricle. The pulmonary and systemic circuits are in parallel rather than in series.
(90% of users answered this question correctly) Edit Explanation 42. E
Chloroform and diethyl ether are organic solvents that break down lipid membranes, such as viral envelope (choice E). Enveloped viruses, like HIV-1, acquire their envelopes from the plasma membrane as they exit the cell they infected. This membrane is largely made up of lipids. Naked viruses, like HAV, lack this lipid viral envelope, and are therefore resistant to chloroform and diethyl ether. An important way of classifying viruses is whether or not they have an envelope.
The other answer choices do not affect whether the virus is sensitive to organic solvents.
(79% of users answered this question correctly) Edit Explanation 43. C
Hyperthyroidism (choice C) is often due to excessive production of thyroid hormone in the absence of TSH stimulation. Classic symptoms include irritability, nervousness, weight loss, increased appetite, tremor, sweating, and tachycardia.
Congestive heart failure (choice A) is a condition where decreased cardiac output causes inadequate perfusion of tissues. It does not cause irritability.
Cushing syndrome (choice B) is any condition that causes excess serum cortisol. Signs and symptoms include weight gain, round face, thinning of skin, purple striae, and hirsutism.
Mitral valve prolapse (choice D) is a condition where the mitral valve bulges into the left atrium during systole. It would not cause this man's clinical presentation.
Pheochromocytoma (choice E) is a tumor of the adrenal medulla that causes excess secretion of catecholamines, such as epinephrine and norepinephrine.
(84% of users answered this question correctly) Edit Explanation 44. C
Gap junctions (choice C) allow transfer of chemical and electrical signals between cells. The key structural component of gap junctions is the connexon, a hollow cylinder made up of transmembrane proteins. Gap junctions allow cells to coordinate activities with each other, and are found in nerve cells, cardiac muscle, smooth muscle, and liver cells.
Basal lamina (choice A) is a layer of extracellular matrix on which the epithelium sits.
Desmosomes (choice B) are structures that bind to intermediate filaments between adjacent cells in order to hold them together.
Glycosaminoglycans (choice D) are the carbohydrate portions of proteoglycans, or mucopolysaccharides, that form the major structural components of the extracellular matrix.
Tight junctions (choice E) are barriers that form a belt-like structure encircling epithelial cells along their apical surface. They prevent movement of small molecules between the two cells.
(82% of users answered this question correctly) Edit Explanation 45. A
This patient's PCO2 and pH are both below normal, indicating a metabolic acidosis. Respiratory acidosis is cause by an increase in PCO2. Therefore answer choices B and E are incorrect.
The anion gap estimates the amount of unmeasured anions in the plasma. This number is obtained by subtracting measured anions (Cl- and HCO3-) from the measured cations (Na+).
Anion gap = 140 - (98+17) = 25 mEq/L
The normal value for the plasma anion gap is 8-16 mEq/L. Therefore this patient's anion gap is higher than normal.
High anion gap metabolic acidosis is caused by an accumulation of an organic anion (for example, ketoacid). Some possible causes for high anion gap metaboilc acidosis include diabetic ketoacidosis, lactic acidosis, and salicylate poisoning.
46. E
The urachus is a connection between the dome of the bladder and the umbilicus that obliterates during development. A urachal cyst (Answer E) is usually located in between the obliterated ends of this developmental duct. A hydrocele (Answer A) is a benign collection of peritoneal or scrotal fluid in the scrotum. It is usually due to patent processus vaginalis or scrotal fluid imbalance. A meckel cyst (Answer B), aka duplication cyst, often contains ectopic gastric mucosa. It is most often located in the ileum, duodenum or stomach. A Meckel's diverticulum (Answer C) is an intestinal outpouching that usually occurs in the ileum. It occurs when the vitelline duct, which connects the primary midgut and the yolk sac, fails to obliterate. During the 6th week of development, the midgut herniates through the umbilical cord. An omphalocele (Answer D) occurs during week 12 when this hernation fails to retract, leaving an amnion-covered sac protruding through the umbilicus.
(83% of users answered this question correctly) Edit Explanation 47. B
Agranulocytosis (Answer B) is a rare adverse effect of all antipsychotics. The RBC (Answer A and E), platelet (Answer C) and eosinophil (Answer D) lines are not known to be affected by clozapine.
Clozapine is a (new-generation) atypical anti-psychotic that blocks 5-HT2 and dopamine receptors. Unlike the traditional antipsychotics, it is useful for treatment of the negative symptoms of schizophrenia. Its major side effect is agranulocytosis, which requires WBC monitoring.
Haloperidol is a high-potency dopamine blocker used to treat psychosis. Drugs in this class are more selective for dopamine receptors compared to low-potency antipsychotics such as chlorpromazine or thioridazine, hence haloperidol is the drug of choice to avoid cross-reactivity at muscarinic, adrenergic, and histaminic receptors. Long-term adverse effects of haloperidol include parkinsonian features and tardive dyskinesia secondary to blockade of the nigrostriatal pathway.
48. C
Light touch and pain sensations are divided by dermatomes throughout the body. Dermatomes of the head are supplied by the 3 branches of the trigeminal nerve anteriorly (most of the face) and the second cervical nerve posteriorly (scalp) (Answer C). Compression of C2 will cause pain and numbness in the occipital region of the scalp. Injury to the cervical spinal cord (Answer A) will cause sensory and motor deficits to the dermatomes and muscles innervated by C1-C8. The opthalmic division of the trigeminal nerve (Answer B) carries sensation from the medial scalp, periorbital regions, and most of the nose. The spinal accessory nerve (Answer D) provides motor innervation to the sternocleidomastoid and trapezius. The vertebral artery (Answer E) branches supply the spinal cord, meninges, and join the circle of willis. Injury to this nerve will cause many more neurological deficits.
49. A
Nasal decongestants are alpha1-adrenergic stimulants that cause smooth muscle contraction, reducing vascular congestion. Benign prostatic hyperplasia is partly due to proliferation of smooth muscle in the bladder neck, urethra, and prostate. Therefore alpha1 blockade is one of the main symptomatic treatments to relieve smooth muscle tension. Main effects of beta2 adrenergic stimulation (Answer B) include bronchodilation and vasodilation. Both sympathetic and parasympathetic ganglionic receptors are nicotinic (Answer C), hence the complex autonomic effects of nicotine. Nicotinic receptor stimulation at the neuromuscular junction (Answer D) causes skeletal muscle contraction, which is not involved in BPH. Serotonergic receptors (Answer E) are not involved in BPH.
50. A
The chart shows that the osmolarity of the urine/plasma decreases and the urine flow rate increases. Ingestion of 1L water is the only answer choice which would produce a decrease in plasma osmolarity.
Upon ingestion of the water, the decreased plasma osmolarity inhibits osmoreceptors in the anterior hypothalamus, decreasing the ADH secretion from the posterior pituitary. Decreased ADH causes decreased water permeability in the late distal tubules and collecting duct, decreasing the reabsorption of water. Therefore urine flow rate increases and urine osmolarity decreases. In this way, the excess water is removed from the system.
Choice B is incorrect because 200 mL of 0.9 (isotonic) saline solution would increase plasma volume, but would not change plasma osmolarity.
Choice C is incorrect because 200 mL of glucose solution would not decrease plasma osmolarity.
Choice D is incorrect because 200 mL of a 3% (hypertonic) saline solution would not decrease plasma osmolarity.
Choice E is incorrect because injection of ADH would increase urine and plasma osmolarity.
(78% of users answered this question correctly) Edit Explanation 51. B
The closer the toxic dose is to the effective dose, the narrower the therapeutic window. Therefore the ratio of toxic/effective dose is what matters. If it is closer to 1, it has the highest risk of toxicity at effective doses, regardless of what the median effective dose actually is. Examples of drugs that have narrow therapeutic windows include lithium and digoxin.
52. B
Fetal swallowing reduces the amount of amniotic fluid. Congenital gastrointestinal obstructions, such as duodenal atresia (Answer B) cause a buildup of amniotic fluid, or polyhydramnios. Erythroblastosis fetalis (Answer C) occurs when maternal and fetal blood mix, leading to alloimmune hemolysis, usually against Rh antigen. In late pregnancy, most amniotic fluid is composed of fetal urine. Renal anomalies that reduce the amount of urine production, such as horseshoe kidney (Answer D), are associated with oligohydramnios. Congenital heart disease does not manifest until after birth (Answer A). Neural tube defects (Answer E) usually do not affect the amount of amniotic fluid.
(80% of users answered this question correctly) Edit Explanation 53. D
Always associate mitral stenosis with rheumatic fever. Other valves can be involved but the most common cause of mitral stenosis is aseptic scarring after systemic Group A Streptococcus infection. The murmur of mitral stenosis is described as above. As with any murmur, as the severity increases so does pressure in the chamber before the valve (in this case the left atrium). Dyspnea, fatigue, and rales on exam are symptoms of congestive heart failure, regardless of the underlying cause. This is also the murmur that may be revealed during pregnancy. Aortic regurgitation is a diastolic murmur characterized by wide pulse pressure and may be associated with aortic dissection or trauma. Aortic stenosis usually occurs in elderly patients secondary to calcifications, but it can also be secondary to congenital bicuspid aortic valve in young people. The aortic stenosis murmur is crescendo decrescendo and radiates to the carotids. Mitral regurgitation murmurs are systolic and can occur secondary to chordae tendinae rupture after MI or in predisposed people with mitral valve prolapse. It is a blowing mid to late systolic murmur. Pulmonic regurgitation is a diastolic murmur best heard in the left 2nd intercostal space usually secondary to congenital defect.
54. B
Self-mutilation is a classic symptom of Lesch-Nyhan syndrome, a genetic HPRT enzyme deficiency. Purines, namely guanine and hypoxanthine, are recycled to GMP and IMP by HPRT (Answer D). A deficiency in HPRT (Answer B) forces purines to be degraded to excess uric acid. Another purine salvage pathway deficiency, APRT deficiency (Answer A), leads to toxic buildup of adenosine metabolites in B and T cells, one of the causes of SCID. Answer C describes nucleic acid recycling, which should prevent uric acid accumulation. PRPP (Answer E) is a purine synthesis enzyme; PRPP superactivity, not deficiency, would cause uric acid buildup.
(83% of users answered this question correctly) Edit Explanation 55. C
Muscle atrophy caused by disuse is usually reversible. It is characterized by a decrease in the density of myofibrils (Answer C) but no change in the overall number of muscle fibers (Answer B). Slow-twitch muscle fibers, fast-twitch fibers and intermediate fibers are differentiated cell types, they do not convert after embryogenesis (Answer A). Long-term exercise causes reactive muscle hypertrophy that may include an increase in mitochondrial content (Answer D). Sarcopenia occurs in old age with failure of satellite cells (Answer E) that help regenerate muscle fibers.
(79% of users answered this question correctly) Edit Explanation 56. D
The patient described has multiple myeloma, which is characterized by proliferation of plasma cells and hyperproduction of monoclonal light chains (Bence-Jones proteins). Accumulation at the renal tubules causes renal insufficiency. MM also causes bone destruction, both from marrow expansion and stimulation of osteoclasts. This causes hypercalcemia, which also contributes to renal dysfunction. As with any leukemia, other cell lines are suppressed, causing anemia and thrombocytopenia.
(69% of users answered this question correctly) Edit Explanation 57. E
Recall that that the sympathetic nervous system causes pupilary dilation (mydriasis) via α-adrenergics, while the parasympathetic nervous system constrics the pupil (miosis) via muscarinic cholinergics.
The diagram in this questions shows that the left pupil is smaller than the right pupil without treatment, which points to either increased parasympathetic stimulation or decreased sympathetic stimulation in the left eye. Adding tyramine, which is an indirect sympathomimetic, has no effect on the left eye, which is consistent with sympathetic denervation of the left eye (choice E).
The treatment with epinephrine had a large effect on the left eye, however, which rules out blockade of alpha-adrenergic receptors (choice A), since blocking the α-adrenergic receptors would have blocked any effect of epinephrine.
Blockade of β-adrenergic receptors (choice B) is incorrect since B-receptors are not involved in pupillary constriction and dilation.
Blockade of muscarinic receptors (choice C) is incorrect because the left eye is constricted without treatment, not dilated as it would be with unopposed sympathetic stimulation.
Finally, inhibition of cholinesterase (choice D) is incorrect because it would not have blocked tyramine from dilating the left eye, since it acts on adrenergics, not cholinergics.
(54% of users answered this question correctly) Edit Explanation 58. E
This patient is most likely taking a statin. Statins inhibit the activity of HMG CoA reductase (choice E). One of the major side effects reported from statin use is myalgia. This is an important presenting symptom because continued use can result in rhabdomyolysis and acute renal failure. Decreasing the production of LDL (choice C) is an effect of statins, but not the direct action.
Bile acid binding resins (choice A) such as cholestyramine do not cause myalgias.
Decreasing the binding of bile acids (choice B) and decreasing the activity of lipoprotein lipase (choice D) are not current methods for lowering serum cholesterol.
(88% of users answered this question correctly) Edit Explanation 59. D
This patient most likely has familial hypercholesterolemia, an autosomal dominant disorder associated with a deficiency in LDL receptors (choice D). The rise in his cholesterol concentration is most likely due to a decrease in LDL receptor-mediated endocytosis of cholesterol-rich LDL. Tendon xanthomas are a typical feature of LDL receptor deficiency. In contrast, yellow papular skin lesions, or eruptive xanthomas, are a typical feature of a mutation in the VLDL receptor gene (choice E) and apoC2 (choice B).
A mutation in apoA2 (choice A) would interfere with HDL cholesterol and would not lead to skin manifestations.
A mutation in apoE4 (choice C) would cause an increase in both triglycerides and cholesterol and there may be yellow deposits in the skin creases on the palms on physical exam.
(89% of users answered this question correctly) Edit Explanation 60. E
The first thing you need to recognize to answer this question is that this patient has hyponatremia (110 mEq/L, normal is between 135-145). Given the clinical finding of a lung mass, the most likely explanation is that this patient has SIADH, a paraneoplastic complication of small cell carcinoma of the lung (choice E). SIADH is syndrome of inappropriate antidiuretic hormone (ADH) secretion, in which the body, or in this case the lung tumor, secretes ADH that is not responsive to normal feedback mechanisms, resulting in inappropriate retention of water and dilution of the sodium concentration.
The other malignancies in the answer choices are not associated with SIADH. Adenocarcinoma of the lung (choice A) is more common in non-smokers and is not associated with SIADH.
Craniopharyngioma (choice B) is usually a tumor of childhood that is a remnant of Rathke’s pouch; it could compress the pituitary but this would lead to decreased ADH production, not increased.
Medullary carcinoma of the thyroid (choice C) could secrete excess calcitonin, but not ADH.
Renal cell carcinoma (choice D) can secrete excess erythropoietin, leading to polycythemia, or excess red blood cells, but this would have no effect on sodium concentration.
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