USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 611:

  Which of the following is a true statement regarding renal clearance of a substance?

  A. is measured in milligram per minute

  B. glucose is greater than that of insulin in the normal kidney

  C. insulin is zero

  D. PAH remains constant regardless of the plasma level

  E. potassium clearance will increase after an injection of aldosterone

  Correct Answer: E

  Section: Physiology Choice E is correct since the clearance of potassium is normally regulated by the action of aldosterone on the distal tubule exchange of sodium reabsorption with potassium secretion. Choice Ais incorrect because renal clearance represents the volume of plasma completely cleared of a substance per unit time, hence the units for renal clearance are milliliter per minute, and not milligram per minute. Choice B is incorrect because a normal kidney reabsorbs 100% of the filtered glucose, so glucose clearance is zero. By contrast, a significant amount of inulin is usually cleared by the kidney. Choice C is incorrect because although inulin is not secreted by the tubule as is PAH, inulin is filtered but not reabsorbed and thus a normal kidney will clear approximately 20% of the inulin from the plasma. Choice D is incorrect because PAH is actively secreted by the proximal tubule. Above a certain plasma level, PAH secretion no longer can increase and thus clearance (calculated as the ratio of urine to plasma concentration multiplied by urine flow) will decline as plasma PAH concentration increases faster than urine PAH concentration.

• Question 612:

  Heparin is a rapidly acting, potent anticoagulant that has many important clinical uses. Which of the following is an action of heparin?

  A. activates prothrombin

  B. acts with antithrombin to inhibit thrombin activity

  C. decreases prothrombin time

  D. inhibits calcium action

  E. promotes vitamin K activity

  Correct Answer: B

  Section: Physiology Thrombin is a critical enzyme in the coagulation cascade. It not only can activate factors VIII and V, it also acts on fibrinogen to form fibrin. Thrombin is essential for clot formation. Antithrombin III modulates the coagulation cascade, serving to inhibit thrombin activity. Heparin acts as an anticoagulant because it accelerates the action of antithrombin III. Heparin does not stimulate vitamin K activity (choice E) and, in any case, anything that promoted vitamin K activity would increase coaguability, since vitamin K is necessary for synthesis of multiple coagulation factors. Activation of prothrombin (choice A) promotes coagulation. Though calcium is an essential cofactor for coagulation, heparin does not act via calcium inhibition (choice D). Because heparin inhibits thrombin action, it would prolong prothrombin time (choice C).

• Question 613:

  Altered plasma renin levels can occur in both normal and pathologic conditions. Which of the following states is associated with a decrease in plasma renin levels?

  A. heart failure

  B. primary aldosteronism

  C. renal artery stenosis

  D. salt restriction

  E. upright posture

  Correct Answer: B

  Section: Physiology Most patients with primary aldosteronism (Conn syndrome) have an adrenal adenoma. The increased plasma aldosterone concentration leads to increased renal Na+ reabsorption, which results in plasma volume expansion. The increase in plasma volume suppresses renin release from the juxtaglomerular apparatus and these patients usually have low plasma renin levels. Secondary aldosteronism is due to elevated renin levels and may be caused by heart failure (choice A) or renal artery stenosis (choice C). Salt restriction (choice D) and upright posture (choice E) decrease renal perfusion pressure and therefore increase renin release from the juxtaglomerular apparatus.

• Question 614:

  Exhibit #1:

  

  Exhibit #2:

  

  Below figure shows the change in the membrane potential of a postsynaptic neuron caused by addition, at the arrow, of a neurotransmitter. The resting membrane potential of this cell is -80 mV. Relevant equilibrium potentials for +, , +, , and are, respectively, +120 mV, +60 mV,

  

  0 mV, -80 mV, and -90 mV. This neurotransmitter likely increases the conductance of which of the following ions?

  A. A

  B. B

  C. C

  D. D

  E. E

  Correct Answer: C

• Question 615:

  Below figure shows an intracellularly recorded action potential from a mammalian neuron. Which of the following statements is correct concerning events taking place at the point in time indicated by the arrow?

  

  A. calcium conductance is higher than its normal resting level

  B. chloride conductance is higher than its normal resting level

  C. potassium conductance is higher than its normal resting level

  D. sodium conductance is higher than its normal resting level

  E. sodium-potassium ATPase activity is depressed

  Correct Answer: C

  Section: Physiology The neuron, at the arrow, is in the refractory period following the action potential. The membrane potential is more negative (hyperpolarized) than its normal resting level because potassium conductance is elevated. The after hyperpolarization is also due to the fact that sodium channels are inactivated causing lower than normal sodium conductance (choice D). In skeletal muscle neither calcium (choice A) nor chloride (choice B) conductances should be elevated. After the action potential, intracellular sodium concentration could be slightly elevated due to the sodium influx that occurred during the action potential. This could stimulate the Na, K-ATPase (choice E).

• Question 616:

  A patient presented with an acute abdomen including fever, marked abdominal distension, acidosis, and leukocytosis. Laparoscopy revealed that large parts of the small intestine were necrotic and as a consequence, the entire ileum of the patient was resected. It is expected that very soon after the surgery the patient will have considerable problems resulting from the malabsorption of which of the following?

  A. iron

  B. bile acids

  C. sodium

  D. vitamin

  E. protein

  Correct Answer: B

• Question 617:

  below figure shows a gastric parietal cell with selected membrane transporters and channels. What do X, Y, and Z represent in the figure?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  E. Option E

  Correct Answer: A

• Question 618:

  In patients with type 4 renal tubular acidosis (RTA) aldosterone deficiency is often a prominent finding. Distal tubular transport of which of the following ions will be affected in these patients?

  A. hydrogen and potassium in exchange for sodium

  B. hydrogen only

  C. potassium only

  D. sodium and bicarbonate

  E. sodium only

  Correct Answer: A

  Section: Physiology The RTArefers to related conditions that are disorders of urine acidification even though other renal functions are not impaired. In type 4 RTA, distal nephron dysfunction is due to either inadequate aldosterone production or aldosterone resistance resulting from intrinsic renal disease. Thus, patients develop hyperchloremic acidosis with hyperkalemia, due to impaired distal tubular secretion of both potassium and hydrogen ions. Treatment of patients is directed at controlling serum potassium. Choices B, C, D, and E are inconsistent with the known actions of aldosterone.

• Question 619:

  Which of the following diseases will result in left ventricular hypertrophy?

  A. Aortic valve stenosis

  B. mitral valve stenosis

  C. pulmonary valve regurgitation

  D. tricuspid valve regurgitation

  E. tricuspid valve stenosis

  Correct Answer: A

  Section: Physiology Stenosis of the outlet valve of the left ventricle greatly increases the work of the ventricle in ejecting blood and will result in left ventricular hypertrophy. Mitral valve stenosis (choice B) will impede filling of the left ventricle and will not cause left ventricular hypertrophy. Tricuspid valve regurgitation (choice D), pulmonary valve regurgitation (choice C), and tricuspid valve stenosis (choice E) cause congestion of the right atria and/or right ventricles and impede flow of blood toward the left ventricle and thus will not result in left ventricular hypertrophy.

• Question 620:

  A 28-year-old, 166 cm (5.45 ft) tall woman, weighing 170 kg (375 lbs) successfully underwent biliopancreatic diversion surgery, in which a portion of her stomach was removed and the remaining portion of the stomach was connected to the lower portion of the small intestine (see below figure). What is her prevalence for peptic ulcer disease and for what reason?

  

  A. it is higher due to inflammation caused by the surgical staples

  B. it is higher due to the loss of secretin release

  C. it is lower since chief cells were surgically resected

  D. it is lower since G cells were surgically resected

  E. it is unchanged since the surgery was successful

  Correct Answer: D

  Section: Physiology Peptic ulcers refer to areas of the stomach or duodenal lining which became eroded by stomach acid. Stomach acid is produced by parietal cells which are stimulated by gastrin. Since gastrin-secreting G cells are primarily found in the gastric glands of the distal stomach (gastric antrum), the surgically removed stomach portion, the patient's prevalence for peptic ulcer disease is lower after the surgery, and not unchanged as stated in choice E. However, the surgery is a dramatic event and increases the risk for many GI problems, including symptoms resulting from nutritional deficiencies. Hence, the surgery is only used for morbidly obese people (the patient's body mass index is above 50), who had no success with diet and medication. Inflammation potentially triggered by the surgical staples (choice A) might trigger prostaglandin release which has a protective function of the stomach lining. Although secretin (choice B) is known to suppress gastric acid release by inhibiting gastrin release, the lower number of G cells after surgery does not make this a good choice. Chief cells (choice C) are primarily present in the proximal stomach, the portion that remains after surgery.