USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 681:

  Using laboratory micropuncture technique, blood plasma is collected from both the afferent arteriole and efferent arteriole of a renal cortical glomerulus. Which of the following has the lowest afferent/efferent arteriole concentration ratio?

  A. albumin

  B. chloride

  C. glucose

  D. potassium

  E. sodium

  Correct Answer: A

  Section: Physiology The process of glomerular ultrafiltration creates a tubular fluid that is essentially protein free. Hence, as plasma passes from the afferent arteriole, through the glomerular capillaries to the efferent arteriole, the protein albumin concentration rises as approximately 20% of the fluid is filtered out, leaving the albumin behind, giving an afferent/efferent arteriole concentration ratio of approximately 0.8. By contrast, the glomerular capillary membrane is freely permeable to water and other small particles such as glucose (choice A), chloride (choice B), potassium (choice D), and sodium (choice E), so their concentrations do not change as approximately 20% of water and solute are filtered into Bowman's capsule, giving afferent/ efferent arteriole concentration ratios of 1.0.

• Question 682:

  An adult male becomes hypotensive during surgery because of blood loss. Intravenous administration of 500 ml of which of the following solutions will have the greatest effect in restoring blood volume, and thus blood pressure to normal?

  A. blood plasma

  B. distilled water

  C. hypoosmotic NaCl

  D. isotonic dextrose solution

  E. isotonic saline solution

  Correct Answer: A

  Section: Physiology Intravenous solutions are distributed in the various body fluid compartments based on osmolality and their ability to penetrate the vascular wall and the cell membrane. The proteins in blood plasma will remain within the vascular compartment because of their low permeability across the vessel wall. Thus, 500 mL of blood plasma will remain in the vascular compartment. Water (choice B) passes across the cell membrane and the vascular wall easily. Hence distilled water will distribute itself between cellular and extracellular spaces, meaning only a small portion will actually contribute to blood volume, the other choices (C, D, and E) cross the vascular barrier and distribute partially to the extravascular space and thus are less effective intravascular volume expanders.

• Question 683:

  A 14-year-old boy presents with weight loss and diarrhea. His tongue becomes sore and blistery after eating oatmeal or rye bread, which leads to the diagnosis of celiac disease. The boy and his parents are advised to be sensitive to symptoms of tetany and paresthesias, since they can occur as a consequence of malabsorption of which of the following?

  A. calcium

  B. carbohydrates

  C. fat

  D. iron

  E. water

  Correct Answer: A

  Section: Physiology In patients with celiac disease the protein gluten, which is found in bread, oats, and many other foods containing wheat, barley, or rye, triggers an autoimmune response that causes damage to the small intestine leading to widespread manifestations of malabsorption. Calcium is difficult to absorb, so patients frequently experience symptoms of hypocalcemia such as muscle cramping, titanic contractions, numbness, and tingling sensations. For sensory and motor nerves, calcium is a critical second messenger involved in normal cell function, neural transmission, and cell membrane stability. The nerves respond to a lack of calcium with hyperexcitability. The inability to absorb nutrients such as carbohydrates (choice B) leads to weight loss as experienced by the boy. Fat malabsorption (choice C) is not uncommon in patients with celiac disease as a consequence of greatly diminished absorptive surface. It would manifest in steatorrhea. Iron (choice D), like calcium, is difficult to absorb and may lead to anemia. The general decrease in electrolyte absorption leads to decreased water absorption (choice E). The increased osmotic load in the GI lumen accounts for the boy's diarrhea.

• Question 684:

  The graph in below figure shows the jugular vein pressure curve of this patient. If the first heart sound occurs between letters C and D, which part of the curve represents the atrial contraction?

  

  A. peak A

  B. peak C

  C. peak D

  D. trough B

  E. trough E

  Correct Answer: B

  Section: Physiology below figure shows the jugular vein pressure curve. Pressure in the jugular vein reflects atrial pressure and is highest during the atrial contraction (named the "a wave"). The "v wave" (choice A) represents the rise in atrial pressure before the tricuspid valve opens during diastole. The "y notch" (choice D) is due to the fall in atrial pressure during the ventricular filling phase. The "c wave" (choice C) is due to the rise in atrial pressure produced by the bulging of the tricuspid valve during isovolumetric contraction of the right ventricle. The "x notch" (choice E) coincides with the ventricular ejection phase.

• Question 685:

  The graph in below figure shows the pressurevolume curve of the left ventricle of this patient (shaded area). The pressurevolume curve of a normal subject is shown for comparison (broken lines). Compared to normal, which term best describes this patient's heart?

  

  A. decreased afterload

  B. decreased preload

  C. decreased stroke volume

  D. increased force of contraction

  E. increased preload

  Correct Answer: D

  Section: Physiology

  Note that the end-diastolic volume of this patient is the same as that of a normal subject. Because the

  patient's stroke volume is larger than that of a normal subject, the force of contraction must also be larger.

  This could be due to increased sympathetic tone or to the fact that the patient took inotropic medications.

  The volume remaining after the ventricular contraction is correspondingly smaller compared to normal.

  Preload (choices B and E) equals end-diastolic volume and is the same in the patient and normal subjects.

  Afterload (choice A) is equal to arterial pressure. Both afterload and stroke volume (choice C) are larger in

  this patient compared to normal.

• Question 686:

  A biopsy from a 55-year-old woman with breast cancer was submitted to an estrogen receptor assay. The binding of estrogen to the cancer cells below figure, solid line was plotted in the form of a Scatchard plot and compared to control cells below figure, dotted line). Which of the following would you conclude for the patient's estrogen receptors?

  

  A. they are equal in number and have a higher affinity

  B. they are equal in number and have a lower affinity

  C. they are fewer in number and have a higher affinity

  D. they are fewer in number and have a lower affinity

  E. they are higher in number and have an equal affinity

  Correct Answer: B

  Section: Physiology The slope of the patient's line is decreased compared to normal. This indicates a low affinity of estrogen for the estrogen receptor, which makes choice B correct and choice Aincorrect. The x- intercept is called Bmax and represents the total number of estrogen receptors. This is unchanged in the patient compared to normal so that choices C, D, and E are excluded. The information about the numbers and status of estrogen and progesterone receptors on cancer cells is of high clinical value to determine the efficiency of consequent hormonal therapy.

• Question 687:

  A 71-year-old man was admitted to the hospital after getting very dizzy upon rising from the toilet seat. At that time his pulse was racing and he remembers that his stool looked very different than usual. Over the last 4 or 5 weeks before the incident, the patient self-medicated with high-dose ibuprofen three times a day to control some pain in his hips. What is the mechanism of action for nonsteroidal anti-inflammatory drugs (NSAIDs) to cause gastrointestinal (GI) bleeding?

  A. they inhibit arachidonic acid synthesis

  B. they inhibit bradykinin synthesis

  C. they inhibit cyclooxygenase

  D. they inhibit histamine synthesis

  E. they promote prostaglandin synthesis

  Correct Answer: C

  Section: Physiology NSAIDs inhibit cyclooxygenase and consequently inhibit (not promote, choice E) synthesis of prostaglandins. In the stomach, prostaglandins have a cytoprotective effect through inhibition of acid secretion, enhancement of mucosal blood flow, and stimulation of bicarbonate and mucus secretion. Inhibiting these processes can cause stomach ulcers and bleeding such as described in the case. Arachidonic acid (choice A) is not directly affected by NSAIDs--it is turned into prostaglandin by the action of cyclooxygenases. NSAIDs do not regulate bradykinin and histamine synthesis (choices B and D).

• Question 688:

  A 52-year-old man has a history of anginal pain that until recently was responsive to nitrates. He is now evaluated for possible angioplasty. The graph in below figure shows the ECG of this patient. Blood flow across the mitral valve is largest around which indicated point in this ECG tracing?

  

  A. A

  B. B

  C. C

  D. D

  E. E

  Correct Answer: E

  Section: Physiology The most rapid filling of the ventricles occurs in early diastole, immediately after opening of the atrioventricular valves. This happens after the repolarization phase (T wave) and resulting relaxation of the cardiac ventricular muscle. Excitation of the atria (choice A) also results in increased blood flow into the ventricles, occurring around choice B. However, the flow at that time is less than during early diastole. Ventricular contraction begins with the QRS complex (choice C) and lasts until the end of the T wave (choice D). During this time, the mitral and tricuspid valves are closed.

• Question 689:

  A 41-year-old male suffering from unstable angina is undergoing cardiac testing. At rest, his heart rate is 70 beats per minute. His left ventricular end-diastolic volume is estimated to be 150 mL, while his end systolic volume is 80 mL. What is his estimated cardiac output?

  A. 3900 mL/min

  B. 4400 mL/min

  C. 4900 mL/min

  D. 5400 mL/min

  E. 5900 mL/min

  Correct Answer: C

  Section: Physiology

  Cardiac output is calculated by multiplying heart rate by stroke volume. Stroke volume is the difference

  between end-diastolic ventricular volume and end-systolic volume. Hence 150 - 80 = 70 mL stroke volume.

  70 mL × 70 beats/min = 4900 mL/min.

• Question 690:

  A dislocated shoulder of a middle-aged man was corrected under conscious sedation with etomidate. Since etomidate has been reported to affect adrenocortical function as a side effect, the patient's ACTH and cortisol were measured in blood samples taken at the indicated times on the following day. What do the ACTH results indicate?

  

  A. inverted circadian pulsatile release

  B. normal circadian pulsatile release

  C. normal circadian non-pulsatile release

  D. primary adrenal insufficiency

  E. secondary adrenal insufficiency

  Correct Answer: A

  Section: Physiology The differences in the hormone concentrations of blood samples taken half-an-hour apart indicate pulsatile release. Hence, non-pulsatile release (choice C) is excluded. However, the normal circadian rhythm for ACTH (choice B) is likely to be highest early in the morning, with cortisol production following ACTH by a short delay, opposite to the patient's values. Such a reversed pattern could, for instance, be observed for night workers and is not likely to be caused by etomidate. Etomidate has been associated with suppression of glucocorticoid and mineralcorticoid synthesis in the adrenal cortex. Substantial damage to the adrenal gland might indeed lead to adrenocortical hypofunction, called Addison disease. However, primary adrenal insufficiency (disorder of the gland, choice D), and secondary adrenal insufficiency (inadequate ACTH secretion, choice E) both present with low levels of cortisol.