USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 651:

  The terminal ileum was removed from a 50-year-old woman during excision of a tumor. About 3 years later, the patient was admitted to the hospital. She is very pale. Hemoglobin is 9 g/dL, MCV (mean corpuscular volume) has increased to 110 (110 fL). The provisional diagnosis is a vitamin

  

  deficiency. Which of the following vitamins is the most likely one causing the symptoms?

  A. A

  B. B1

  C. B6

  D. B12

  E. K

  Correct Answer: D

• Question 652:

  A 23-year-old Caucasian male is admitted to the hospital following a motorcycle accident. On examination, no bones appear to be broken, but there is extensive muscle bruising resulting in tissue swelling from increased capillary permeability. His arterial blood pressure is 80/40. He is awake and able to walk with assistance. Based on this information, it is likely that which of the following will be decreased from normal?

  A. circulating levels of catecholamines

  B. left atrial pressure

  C. plasma aldosterone concentration

  D. plasma lactate concentration

  E. plasma renin activity

  Correct Answer: D

  Section: Physiology The accident trauma produces extensive loss of fluid from the vasculature into the interstitial fluid space. Hence venous return to the heart, and thus left atrial pressure will be reduced. The decrease in arterial pressure will trigger arterial baroreflexes which will activate the sympathetic nervous system and increase catecholamine release (choice A). Likewise the fall in pressure will cause increased renin/angiotensin/ aldosterone (choices C and E). Finally, the reduced perfusion of the tissues will generate increased lactate formation (choice D).

• Question 653:

  Exogenous peripherally injected insulin differs from endogenously secreted insulin in a number of aspects, including which of the following?

  A. achieves a higher concentration in the periphery than in the liver, contrary to endogenous insulin

  B. contains C-peptide, which is missing from secreted endogenous insulin

  C. is able to bypass insulin resistance observed with endogenous insulin

  D. is always extracted from animal sources and, therefore, is less effective due to sequence differences and anti-insulin antibodies

  E. is in the form of proinsulin, whereas endogenous insulin has had C-peptide removed

  Correct Answer: A

  Section: Physiology The concentration of exogenous insulin is higher at the site of injection in the periphery, compared to its concentration in the liver. On the other hand, endogenous insulin is higher concentrated in the liver than in the periphery. Insulin is normally secreted by the endocrine pancreas into the portal venous drainage. Thus, it passes through the liver before being seen by the periphery. Acertain fraction of insulin is extracted by the liver; so that the concentration of insulin seen by the liver is normally higher than that seen by the periphery. This discrepancy between exogenous and endogenous insulin might contribute to the problems experienced by diabetics such as hypertension and cardiovascular disease. Injected insulin is missing C- peptide (choice B), while endogenous insulin is cosecreted with C- peptide, the physiologic functions of which remain unknown. Injected insulin is biologically active and not in the proinsulin form (choice E). Exogenous insulin generally used today in the United States is recombinant human insulin, not from animal sources (choice D). The problem of developing anti- insulin antibodies is now rarer since the injected insulin is not from animals. Insulin resistance (choice C) is generally due to receptor downregulation or desensitization, or anti-insulin antibodies. Tissues become equally resistant to both the body's own (endogenous) and administered (exogenous) insulin.

• Question 654:

  Evaluating the cause for a coma can be challenging. As a first step, the causes can be divided into two broad categories, structural/ surgical and metabolic/medical reasons. Which of the following findings suggests a metabolic cause in a comatose patient?

  A. failure to withdraw from painful stimuli

  B. gross blood in the cerebrospinal fluid

  C. impaired pupillary light responses

  D. posturing of limbs

  E. serum sodium of 115 meq/L

  Correct Answer: E

  Section: Physiology Hyponatremia of this magnitude for any reason is likely to cause seizures and coma, especially if the deviation from normal occurred rapidly (e.g., over 1 or 2 days). Withdrawal from painful stimuli (choice A) is a normal reaction. If intact, it would have provided information on the intactness of sensation and motor reflexes on the tested side. However, lack of withdrawal would be consistent with either a structural or a metabolic cause of coma, and therefore does not help. Blood in the cerebral spinal fluid (choice B) in a nontraumatic lumbar puncture suggests a structural rather than a metabolic cause of coma. Pupillary light responses (choice C) are typically preserved in metabolic encephalopathies. Posturing of limbs (choice D) is suggestive of a focal process involving the brainstem or midbrain rather than a metabolic encephalopathy.

• Question 655:

  A 35-year-old weight lifter, who has been injecting testosterone for muscle mass augmentation, is evaluated for sterility and found to have an extremely low sperm count. Which of the following is an effect of testosterone and contributes to the mentioned sterility?

  A. activation of inhibin

  B. feedback activation of leptin

  C. feedback inhibition of GnRH

  D. inhibition of seminal prostaglandins

  E. lowered core temperature

  Correct Answer: C

  Section: Physiology Testosterone directly inhibits the secretion of GnRH from the hypothalamus, which affects secretion of LH and FSH and consequently secretion of testosterone. To initiate spermatogenesis, both FSH and testosterone are necessary. To maintain spermatogenesis after puberty, extremely high concentrations of testosterone seem to be required. Systemically administered testosterone does not raise the androgen level in the testes to as great a degree and it additionally inhibits LH secretion. Consequently, the net effect is generally a decrease in sperm count. There are two forms of inhibin (choice A) that are produced by Sertoli cells in males. They are activated by FSH, not testosterone, and inhibit FSH secretion by a direct action on the pituitary. Testosterone has been shown to have a suppressive, not activating (choice B) effect on leptin production. Prostaglandins (choice D) are produced by seminal vesicles and found in large quantities in semen. They are not known to play a role in spermatogenesis. They are believed to aid fertilization by reacting with female cervical mucus and to support smooth muscle movements in the uterus and fallopian tubes. Testosterone stimulates basic metabolic rate and can raise, not lower body core temperature (choice E). Excessive temperature of the testes can temporarily cause sterility.

• Question 656:

  A 59-year-old Caucasian female is self-donating blood in preparation for a hip replacement surgery in the near future. Shortly after her third session of donating a unit (pint) of whole blood, her mean arterial pressure remains unchanged, even though the venous return of blood to her heart is diminished. Which of the following is the most likely reason for the preservation of arterial pressure?

  A. cardiac output rises to compensate for the reduced venous return

  B. end-diastolic ventricular filling pressure rises during hemorrhage

  C. fall in venous return is offset by an increase in total peripheral resistance

  D. heart rate rises to compensate for a reduced venous return

  E. venous return and blood pressure are unrelated

  Correct Answer: C

  Section: Physiology By Ohm's law, a reduction in flow should also reduce pressure if resistance is constant. Therefore a rise in total peripheral resistance in the face of reduced flow would account for the preservation of arterial pressure. Choice A is incorrect since cardiac output and venous return must rise and fall together. Choice B is incorrect since end-diastolic ventricular filling pressure falls with decreased venous return. Choice D is incorrect since heart rate can maintain pressure only if venous return is also maintained. Choice E is incorrect since Ohm's law states the inverse relationship of venous return and pressure.

• Question 657:

  A 4-year-old child with signs of precocious (early onset) puberty is brought to a clinic for evaluation and found to have a congenital deficiency of 21--hydroxylase. Feedback inhibition of the pituitary gland is lost and excess ACTH is secreted. As a result, which of the following happens?

  A. adrenal cortical atrophy occurs

  B. adrenal medullary hypertrophy occurs

  C. excess cortisol is released

  D. precursors to cortisol synthesis increase

  E. serum cholesterol falls dramatically

  Correct Answer: D

  Section: Physiology In the adrenogenital syndrome, the failure to make cortisol due to lack of the adrenal enzyme 21-hydroxylase results in an inability to provide negative feedback suppression of ACTH production. As a result, the adrenal glands are under constant stimulation to maximize steroidogenesis. Substrates that cannot reach cortisol flow down other pathways and by mass action drive the massive overproduction of androgens, which can also be peripherally aromatized to estrogens. No significant change in serum cholesterol is observed (choice E), probably because the cholesterol reservoir in the body is large, even compared to the massive levels of steroids being synthesized in this syndrome. Cortical atrophy (choice A) and release of excess cortisol (choice C) are the opposite of what is observed. There is no mechanism to achieve selective hypertrophy of the adrenal medulla (choice B) because the action of ACTH to drive adrenal hypertrophy is limited to the cortex.

• Question 658:

  A 30-year-old male seeks help because he lost weight and feels full after eating only a small amount of food. He is diagnosed with a delay in gastric emptying. Which of the following hormones has at physiological levels the strongest effect in inhibiting gastric emptying?

  A. cholecystokinin

  B. gastrin

  C. glucose-dependent insulinotropic peptide

  D. motilin

  E. pancreatic polypeptide

  Correct Answer: A

  Section: Physiology The major control mechanism for gastric emptying involves duodenal gastric feedback, hormonal as well as neural. The major hormone involved in the inhibition of gastric emptying is cholecystokinin (CCK), which is released by fat and protein digestion products. Gastrin (choice B) stimulates hydrochloric acid secretion and exerts a trophic effect on the gastric and intestinal mucosa. When the gastrin concentration is elevated to supraphysiologic levels, various other actions can be demonstrated including inhibition of gastric emptying. However, the physiologic importance of these actions is uncertain. Glucose-dependent insulinotropic peptide (GIP, choice C) is released from the intestinal mucosa by acid, fat, or hyperosmolarity and acts to some extent by inhibiting stomach functions including gastric motility. Although this function gave GIP its initial name (gastric inhibitory peptide), GIP's action as an enterogastrone is now controversial. The major physiologic action of GIP is to cause insulin release. Motilin (choice D) stimulates gastric motor activity, especially during the interdigestive phase, when it regulates contractions that serve to empty the GI residual contents. Pancreatic polypeptide (choice E) is a negative feedback regulator for pancreatic enzymes and bicarbonate secretion. It is considered to be a candidate hormone since it satisfies some, but not all of the criteria for hormonal status.

• Question 659:

  A patient with chronic renal insufficiency due to renal vascular disease has a net functional loss of nephrons. If we assume that production of urea and creatinine is constant and that the patient is in a steady state, a 50% decrease in the normal GFR will cause which of the following to occur?

  A. decrease plasma urea concentration

  B. greatly increase plasma

  C. increase the percent of filtered Na+ excreted

  D. not affect plasma creatinine

  E. significantly decrease plasma

  Correct Answer: C

• Question 660:

  A 53-year-old man is being treated for hypertension and diabetes. His medications include insulin and propranolol. He presents at his physician's office complaining of muscle weakness. Blood tests reveal hyperkalemia (elevated serum potassium) as well as elevated BUN (blood urea nitrogen). Propranolol is gradually eliminated and his insulin dosage is adjusted. His serum potassium normalizes and his muscle weakness is alleviated. What probably caused his muscle weakness?

  A. high potassium-mediated block of acetylcholine receptors

  B. high potassium-mediated block of skeletal muscle calcium channels

  C. motor neuron hyperpolarization

  D. skeletal muscle depolarization with resultant Na-channel inactivation

  E. skeletal muscle hyperpolarization with resultant Na-channel blockade

  Correct Answer: D

  Section: Physiology Elevated serum potassium levels cause membrane depolarization with a resulting Na-channel inactivation. Fibers are thus less able to fire action potentials, leading to impaired excitation contraction coupling, with muscle weakness (choice D is correct). Though hyperpolarization would also impede action potential generation, by moving the Na-channel away from its activation threshold, choices C and E are incorrect since high potassium causes membrane depolarization. Calcium required for skeletal muscle contraction is derived from internal stores (the sarcoplasmic reticulum) and is not dependent on calcium influx through surface membrane channels (choice B). There is no evidence that potassium interferes with the acetylcholine receptor (choice A). Hyperkalemia in this patient is probably due to multiple factors. Since insulin promotes potassium uptake into cells, too low an insulin dosage in the diabetic can lead to hyperkalemia. In addition, propranolol can cause a shift of potassium from cell to blood. Finally, the elevated BUN indicates some renal failure, and failing kidneys cannot efficiently excrete potassium into the urine.