USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 671:

  A 66-year-old woman is undergoing surgery to repair an abdominal aortic aneurism. During the procedure the abdominal aorta is momentarily clamped below the renal arteries. below figure shows the left ventricular pressure/ volume relationship immediately before (curve 1) and during (curve 2) the aortic occlusion. The change in curve 2 represents an increase in which of the following?

  

  A. afterload

  B. cardiac output

  C. ejection fraction

  D. preload

  E. stroke volume

  Correct Answer: A

  Section: Physiology Clamping of the abdominal aorta suddenly increases peripheral vascular resistance to blood flow. Hence aortic pressure suddenly rises to a high level. This represents an increased afterload, that is, an increased ventricular pressure is required to eject blood into the aorta. below figure shows that momentary clamping of the aorta does not alter the preload, that is, the filling of the ventricle during diastole (choice D). below figure shows that the inverse relationship between afterload and stroke volume causes a reduced stroke volume (choice E) immediately after aortic clamping. Since end- diastolic volume (preload) is unchanged, while stroke volume is decreased, this represents a decreased ejection fraction (choice C). Cardiac output is calculated as stroke volume X heart rate. Since heart rate is not given, the cardiac output cannot be determined (choice B). However, it is likely, by Ohm's law that a momentary rise in peripheral vascular resistance will decrease cardiac output.

• Question 672:

  Exhibit:

  

  The action potentials shown in below figure represent those of which kind of cells?

  A. cardiac nodal cells

  B. myelinated motor axons

  C. sensory neurons

  D. skeletal muscle cells

  E. ventricular Purkinje cells

  Correct Answer: A

  Section: Physiology The action potentials illustrated must be those of cardiac nodal cells (SA node or AV node). The duration of these action potentials is too long for motor axons, (choice B), sensory neurons (choice C), or skeletal muscle fibers (choice D). Also, the configuration is different and a pacemaker potential is clearly present. They cannot be ventricular Purkinje action potentials (choice E) because these have a more negative diastolic component that does not gradually depolarize, a longer duration (200 msec) and a plateau region.

• Question 673:

  Below figure shows the amounts of water ingested and secreted daily into the GI tract by a healthy individual. Since about 100 mL of water is excreted daily in the stool, which of the following volumes (in milliliter) best reflects the daily absorbed water in the indicated areas x, y, and z of the GI tract?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  E. Option E

  Correct Answer: B

  Section: Physiology The small intestine absorbs massive quantities of water. Anormal person takes in about 2 L of fluid every day. Another 7 L of fluid are received by the small intestine daily as secretions from salivary glands (1.5 L), stomach (2 L), pancreas (1.5 L), liver (0.5 L), and the small intestine itself (1.5 L). That results in a total of about 9 L of fluid entering the GI tract daily. In a healthy person the duodenum, jejunum, and ileum absorb the bulk of the water that enters the GI tract, here 8500 mL. The colon absorbs most of the remaining fluid (400 mL), not the bulk of it (choice A) and excretes about 100 mL daily. There is generally no water absorption in the esophagus and stomach. Therefore, choices C, D, and E are incorrect.

• Question 674:

  Lipoxygenase converts arachidonic acid to biologically active compounds called leukotrienes. Leukotrienes have been implicated in several disease entities, including allergic asthma, where they are presumed to mediate bronchoconstriction. Introducing leukotrienes into an airway would be expected to cause which of the following responses?

  A. decreased airway resistance

  B. decreased dead space volume

  C. increased functional residual capacity (FRC)

  D. increased lung compliance E. increased total lung capacity

  Correct Answer: B

  Section: Physiology Bronchiole volume contributes to dead space volume, so increasing bronchoconstriction would decrease bronchiolar volume and thus decrease dead space volume. Bronchoconstriction is a major determinant of airway resistance to air flow. Leukotrienes, by increasing airway constriction, would increase resistance to air flow (choice A). Lung elastic tissue and alveolar surface tension determine lung compliance (choice D), which should not be affected by airway leukotrienes exposure. FRC (choice C), the point of mechanical balance between chest wall and lung, is dependent on lung compliance and, similarly, would not be affected by leukotrienes exposure. Any bronchoconstriction would slightly decrease total lung capacity (choice E).

• Question 675:

  Exhibit:

  

  Please refer to the exhibit.

  The majority of in the blood is carried in which of the following forms?

  

  A. A

  B. B

  C. C

  D. D

  E. E

  Correct Answer: D

• Question 676:

  A patient with acute glomerulonephritis has a total plasma of 2.5 mmol/L and a GFR of 160

  

  L/day. What is the estimated daily filtered load of calcium?

  A. 64 mmol/day

  B. 120 mmol/day

  C. 240 mmol/day

  D. 400 mmol/day

  E. 800 mmol/day

  Correct Answer: C

• Question 677:

  Lack of oxygen (hypoxia) will cause reflex vasoconstriction in the circulation supplying which of the following organs?

  A. brain

  B. heart muscle

  C. kidney

  D. lungs

  E. skeletal muscle

  Correct Answer: D

  Section: Physiology Local hypoxia causes vasoconstriction in vessels of the pulmonary circulation. This is an adaptive response, since it will shunt blood away from poorly ventilated alveoli. This response differentiates the pulmonary circulation from the vasculature of other organs. In heart muscle, skeletal muscle, kidney, and brain (choices A, B, C, and E) vasodilation occurs in response to local hypoxia.

• Question 678:

  A 53-year-old healthy male undergoes an exercise stress test, running on a treadmill until a maximum exertion is obtained. Which of the following statements correctly describes effects of autonomic nerve activity on the cardiovascular system in such a healthy subject?

  A. inhibition of parasympathetic nerves decreases total peripheral resistance

  B. inhibition of parasympathetic nerves increases heart rate

  C. inhibition of parasympathetic nerves increases total peripheral resistance

  D. stimulation of parasympathetic nerves decreases the strength of cardiac ventricular contractions

  E. stimulation of sympathetic nerves decreases the strength of cardiac ventricular contractions

  Correct Answer: B

  Section: Physiology Since cardiac SA nodal cells receive tonic input from both sympathetic and parasympathetic nerves, heart rate increases whenever sympathetic firing rate increases or parasympathetic firing rate decreases. In humans, the parasympathetic innervation of ventricular muscle is negligible (choice D), and the strength of contraction increases with increasing preload and with increasing sympathetic firing rate (choice E). With few exceptions, blood vessels are not innervated by parasympathetic nerves, and there is little effect of changes in parasympathetic tone on total peripheral resistance (choices A and C).

• Question 679:

  High-dose glucocorticoid therapy for treatment of rheumatoid arthritis remains highly controversial. It is widely agreed that it is highly effective in controlling acute rheumatoid inflammation, but it may also result in significant adverse effects. Complications of high-dose glucocorticoid therapy include which of the following?

  A. excessive growth in children and acromegaly in adults

  B. hyperkalemia

  C. hyponatremia

  D. suppression of the hypothalamicpituitary-adrenal axis

  E. volume depletion

  Correct Answer: D

  Section: Physiology High-dose exogenous glucocorticoids suppress the adrenal neuroendocrine axis. Patients treated for longer than 2 weeks need to be tapered off glucocorticoids slowly to avoid adrenal insufficiency. Other complications of high-dose glucocorticoids include growth suppression, not excessive growth (choice A) in children and volume overload, not volume depletion (choice E). Hyperkalemia (choice B) and hyponatremia (choice C) are observed in adrenal insufficiency due to loss of mineralocorticoid effects and are not relevant to glucocorticoid therapy.

• Question 680:

  It is known that stretch receptors contained in the walls of the atria convey nerve impulses to the brainstem via the vagus nerve. Under normal conditions, these nerve impulses are most likely to occur during ventricular systole. What information is communicated to the brain by these nerve impulses?

  A. arterial muscle contraction

  B. diastolic arterial pressure

  C. filling of the atrium by venous pressure

  D. filling of the ventricle

  E. systolic arterial pressure

  Correct Answer: C

  Section: Physiology

  The atrial stretch receptors are depolarized by stretch of the atria proportional to the magnitude of venous-

  filling pressure. The frequency of action potentials rises to a maximum at the peak of the atrial "v" wave,

  which signals maximum atrial filling during ventricular contraction. Although the remaining choices (A, B,

  D, and E) may be influenced by the filling of the heart by venous pressure, the information conveyed to the

  brain via the atrial stretch receptors relates directly to the filling pressure of the atria themselves.