USMLE USMLE Certifications USMLE-STEP-1 Questions & Answers

• Question 621:

  The electrocardiogram of a 66-year-old male with a history of atherosclerotic heart disease reveals an irregular, but rapid heart rate. The QRS complexes are normal but no P-waves can be seen. What is the most likely reason for these findings?

  A. atrial fibrillation

  B. Paroxysmal ventricular tachycardia

  C. Right bundle branch block

  D. Sinus tachycardia

  E. Wolff-Parkinson-White syndrome

  Correct Answer: A

  Section: Physiology Atrial fibrillation is a continuous, chaotic reentry of electrical impulses within the atrial myocardium that arises in a diseased or stretched left atrium. The chaotic patterns of atrial excitation prevent P- waves from being seen in the electrocardiogram. The ventricular response is rapid and irregular. In all the other alternatives (choices BE), a P-wave should be distinguishable in some part of the electrocardiogram, although not necessary in a constant relationship to the QRS complex.

• Question 622:

  A newlywed 23-year-old woman and her 28-year-old husband are evaluated for infertility. They have been unable to conceive a child despite regular intercourse for the past 12 months. The first step of this couple's infertility workup is to determine whether ovulation occurs regularly. Which of the following hormones is directly responsible for ovulation?

  A. estradiol

  B. estriol

  C. follicle-stimulating hormone (FSH)

  D. inhibin

  E. luteinizing hormone (LH)

  Correct Answer: E

  Section: Physiology Although the early maturation of an ovarian follicle depends on the presence of FSH, ovulation is induced by a surge of LH. Although estrogens (choices A and B) usually have a negative feedback effect on LH and FSH secretion, the LH surge seems to be a response to elevated estrogen levels. In concert with FSH, LH induces rapid follicular swelling. LH also acts directly on the granulose cells, causing them to decrease estrogen production, as well as initiating production of small amounts of progesterone. These changes lead to ovulation. FSH (choice C) causes follicle maturation, and is also required for Sertoli cells to mediate the development of spermatids into mature sperm cells. Inhibin (choice D) is a polypeptide secreted by the testes and ovaries that inhibits FSH secretion.

• Question 623:

  Exhibit:

  

  The graph in below figure shows the static pressurevolume curve of a patient's lung (solid line). The broken line indicates the pressurevolume curve of a normal person for comparison. What is the approximate lung compliance of this patient over the volume range from 1 to 7 L?

  

  A. A

  B. B

  C. C

  D. D

  E. E

  Correct Answer: C

• Question 624:

  Exhibit #1: Missing Exhibit #2:

  

  The data in below figure indicate that the lung compliance in this patient (solid line) is altered relative to the norm. This patient is most likely suffering from which of the following conditions?

  A. A

  B. B

  C. C

  D. D

  E. E

  Correct Answer: B

• Question 625:

  Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder in which autoantibodies attack voltage-gated calcium channels in the presynaptic nerve terminal. A characteristic manifestation of LEMS is limb muscle weakness. A decrease in which of the following would you expect to see in LEMS?

  A. activation threshold for the action potential

  B. amplitude of the action potential

  C. amplitude of the miniature end-plate potential (MEPP)

  D. magnitude of the end-plate potential (EPP)

  E. quantal content

  Correct Answer: D

• Question 626:

  Which of the following conditions would likely be associated with decreased levels of circulating EPO?

  A. chronic alkalosis

  B. chronic renal failure

  C. emphysema

  D. pernicious anemia

  E. pulmonary fibrosis

  Correct Answer: B

  Section: Physiology EPO is a glycoprotein produced primarily in the kidney, thus EPO levels tend to be depressed in chronic renal failure. EPO is a growth factor that stimulates the production of red blood cells. Its production by the kidney is triggered by low tissue oxygenation. Any condition that decreases the oxygen-carrying capacity of the blood (such as anemia, choice D) or that causes hypoxia by decreasing lung function (such as emphysema or pulmonary fibrosis, choices C and E) will produce elevated levels of circulating EPO. Alkalosis (choice A) increases hemoglobin's affinity for oxygen, making it more difficult for tissues to extract oxygen from the blood. In alkalosis, tissues thus tend to have lower oxygen content, with a resultant increase in circulating EPO levels.

• Question 627:

  Multiple sclerosis is a relatively common nervous system demyelinating disease. It is autoimmune and restricted to the central nervous system. Nerve conduction velocity is depressed in almost all affected individuals. Manipulations which prolong action potential duration seem to mitigate symptoms, possibly by facilitating conduction through sections of membrane which are no longer myelinated. Application of which type of drug might be expected to prolong action potential duration and thus be a potential therapeutic tool?

  A. activates potassium channels

  B. blocks L-type calcium channels

  C. blocks potassium channels

  D. blocks sodium channels

  E. increases sodium channel inactivation

  Correct Answer: C

  Section: Physiology In clinical trials, a class of drugs, the aminopyridines, that blocks certain potassium channels has shown some promise for symptomatic relief of the symptoms of multiple sclerosis. The neuronal action potential is terminated by sodium channel inactivation and potassium channel activation. A drug that blocks potassium channels would thus prolong the action potential. Activating potassium channels (choice A) or increasing sodium channel inactivation (choice E) would shorten the action potential. Blocking sodium channels (choice D) would either shorten the action potential or block it altogether. Since calcium channels are not involved in the neuronal action potential, calcium channel blockade would not be expected to have much effect (choice B).

• Question 628:

  Apatient is found to be deficient in folate. This patient is anemic, and a complete blood count indicates that the MCV is 105 fL (normal range: 8096) and the MCHC is 34 g/dL (normal range: 3236). The anemia is thus macrocytic, normochromic. In this patient how would you predict that the MCH (mean corpuscular hemoglobin) would compare with the normal range?

  A. MCH would be elevated with respect to the normal range

  B. MCH would be depressed with respect to the normal range

  C. MCH would be within the normal range

  D. this cannot be determined based on the information provided

  Correct Answer: A

  Section: Physiology

  These red blood cells are large (macrocytic) but have a normal hemoglobin concentration (normal MCH).

  Since MCH is mean hemoglobin content per red cell, this value must be elevated since the cells are large

  and the concentration in the cell is normal. Choice A excludes choices BD.

• Question 629:

  You are the primary care physician for a young female college student. She has been engaged over the past 3 months in a vigorous aerobic exercise training regime that includes a significant amount of running in a hot environment. You measure her hematocrit as part of a general physical examination. Her hematocrit could well be abnormal even though she is not anemic. Which of the following choices best fits this profile?

  

  A. Option A

  B. Option B

  C. Option C

  D. Option D

  E. Option E

  Correct Answer: E

  Section: Physiology The central points of this question concern the definitions of hematocrit and anemia. Hematocrit is the proportion of blood that is packed cells. The hematocrit reading will be decreased by anything that lowers the number of cells, as well as by anything that increases the volume of plasma. Anemia refers to a condition in which the total circulating erythrocyte mass has decreased. In this student, a long-term regime of exercise in a warm environment has chronically increased her plasma volume, hence her hematocrit is low. However, since her total red blood cell mass is normal, there is no anemia. Choice E is correct. It is common to see a chronic increase in plasma volume develop in response to long-term exercise in a warm environment. Exercising in a warm environment places demands on the circulatory system, which must perfuse working muscles as well as skin capillary beds (to facilitate heat loss). In addition, volume is lost to sweating. Increased plasma volume helps the body meet these demands. In choice A, hematocrit could be low if erythrocyte mass decreased more than blood volume. However, this profile does not fit the question because this individual would be anemic, since erythrocyte mass is low. Choice B could not be correct since decreased plasma volume and increased red cell mass cause increased hematocrit. In choice C, the patient would not be anemic and hematocrit would be increased. However, this is not the best response since, as described above, in an individual exercising in a warm environment the hematocrit is often decreased and plasma volume is usually increased. In choice D, hematocrit would have to be increased.

• Question 630:

  A 66-year-old male presents with angina and dyspnea on exertion. Auscultation of the chest reveals a loud systolic murmur. He is diagnosed with aortic valvular sclerosis. This stenosis of the aortic valve will cause a murmur that can be heard loudest at which interval shown in below figure?

  

  A. between A and B

  B. between B and C

  C. between C and D

  D. between D and A

  E. between D and B

  Correct Answer: C

  Section: Physiology Blood passes through the aortic valve, starting when the aortic valve opens at point C until it closes at point D. Hence aortic valve stenosis will cause turbulent blood flow (murmur) between C and D. Between Aand B (choice A) is the interval of diastolic filling when the aortic valve is closed. Between B and C is the period of isovolumetric contraction (choice B) before the aortic valve opens. Between D and Ais the period of isovolumetric relaxation (choice D) after the aortic valve closes. Between D and B the aortic valve is also closed.