USMLE USMLE-STEP-1 Online Practice Questions and Exam Preparation

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 461:

  A patient is admitted for acute coronary syndrome with signs of impending myocardial infarction. She undergoes angioplasty with double coronary stenting to maintain the patency of her coronary vessels. Which of the following drugs will probably be administered to prevent clotting in the region of the wire mesh stents?

  A. clopidogrel
  B. low-molecular-weight heparin
  C. regular heparin
  D. tissue plasminogen activator (t-PA)
  E. warfarin
  A. clopidogrel

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  Explanation

  Section: Pharmacology Foreign material (such as a stent) in blood vessels invites clotting and the stented vessel will rapidly occlude if clotting is not prevented. The primary process in the formation of this arterial clot is platelet aggregation, so an antiplatelet drug such as clopidogrel is suitable (choice A). Abciximab and tirofiban are also sometimes used in this setting. Heparin (both forms, choices B and C) and warfarin (choice E) are useful in the prevention of venous thrombosis. t-PA (choice D) is useful in accelerating the dissolution of a fibrin clot (e.g., after myocardial infarction), but is not useful in this situation.

• Question 462:

  During surgery to repair a strangulated inguinal hernia, it is discovered that the left testicular vein is thrombosed and must be repaired. The left testicular vein normally drains into which of the following?

  A. inferior vena cava
  B. left renal vein
  C. right common iliac vein
  D. right femoral vein
  E. right renal vein
  B. left renal vein

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  Explanation

  Section: Anatomy The left testicular vein drains into the left renal vein. The inferior vena cava (choice A) receives the right testicular vein. The equivalent relationship holds true in the female where the left ovarian vein drains into the left renal vein and the right ovarian vein into the inferior vena cava. The right common iliac vein (choice C), right femoral vein (choice D), and right renal vein (choice E) do not receive gonadal veins.

• Question 463:

  Which of the following is the best neutrophil and monocyte attractant?

  A. C5a
  B. HLA-A
  C. HLA-B
  D. J chain
  E. variable region of the heavy chain of gG
  A. C5a

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  Explanation

  Section: Microbiology/Immunology C5a is a component of complement. Activation of complement by endotoxin, or antigen-antibody complexes produces C5a, which is a powerful neutrophil, monocyte, eosinophil, and basophil attractant. C5a along with C3a are anaphylatoxins which mediate histamine release from mast cells leading to increased vascular permeability and vasodilation. The variable region of the heavy chain of IgG is not known as the best neutrophil or macrophage attractant (choice E). The J moiety of IgM and IgA does not possess chemotactic properties for neutrophils and macrophages (choice D). HLA-A and HLA-B are genes for the human leukocyte antigens (HLA), and they control the synthesis of class I antigens (choices B and C).

• Question 464:

  In Fig. following figure which labeled bracket spans a sarcomere?

  

  A. A
  B. B
  C. C
  D. D
  E. E
  A. A

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  Explanation

  Section: Anatomy An individual sarcomere, the unit of contraction in striated muscle, spans the interval between successive Z lines. Each sarcomere encompasses an A-band (choice B) and half of each of two I bands (choice D). Each myofibril (choice C) of a striated muscle fiber is composed of a tandem series of sarcomeres. Coupling of excitation and contraction is a critical function of the triad (choice E), which is composed of a T tubule interposed between two cisternae of the sarcoplasmic reticulum.

• Question 465:

  Figure shows plasma concentrations of drug W in a patient following intravenous (IV) injection of a single dose of 10 mg of the drug, an agent eliminated from the body by renal excretion. At hour 12, the patient is treated with sodium bicarbonate (arrow).

  

  Use the data before hour 12 to calculate the volume of distribution (Vd) for drug W.

  A. 1 L
  B. 10 L
  C. 25 L
  D. 50 L
  E. 100 L
  B. 10 L

• Question 466:

  A third-year medical student is asked to review the computerized tomographic (CT) results of a patient with a possible abdominal aortic aneurysm. Which of the following arrows in Following figure points to the abdominal aorta?

  

  A. 1
  B. 2
  C. 3
  D. 4
  E. 5
  B. 2

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  Explanation

  Section: Anatomy Arrow 2 points to the abdominal aorta, located anterior to the lumbar vertebra. Arrows 1 (choice A) and 5 (choice B) indicate the left and right kidneys respectively. Arrow 3 points to the inferior vena cava; the junctions of the left and right renal veins to the inferior vena cava are seen in this CT scan. Arrow 4 points to a loop of the small intestine, filled with contrast-enhancing material.

• Question 467:

  Apatient calls 911 because of severe substernal crushing pain that has not responded to three nitroglycerin tablets taken sublingually. The paramedic team suspects that the patient has suffered an acute myocardial infarction. His blood pressure is 110/70 and his heart rate is 70, with occasional premature extra beats. What drug should be administered immediately, even before the patient reaches the hospital?

  A. aspirin
  B. dobutamine
  C. lidocaine
  D. metoprolol
  E. streptokinase
  A. aspirin

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  Explanation

  Section: Pharmacology Immediate administration of one aspirin tablet by mouth (choice A) has been shown to reduce morbidity and mortality from myocardial infarction. Because this dose is essentially free from adverse effects, this therapy is appropriate even before the diagnosis is confirmed by ECG or other means. Dobutamine (choice B) is a beta-1-selective agonist that has positive inotropic action. It is not appropriate at this time because there is no evidence of heart failure. Lidocaine (choice C), an antiarrhythmic, is not appropriate at this time because the patient is in sinus rhythm with only occasional arrhythmic beats. Metoprolol (choice D), a betablocking drug, is often used after the acute period of a myocardial infarction, but is inappropriate until the patient's condition is stabilized. Streptokinase (choice E), a thrombolytic agent, is inappropriate because coronary thrombosis has not been confirmed and this IV drug is given in the hospital setting.

• Question 468:

  Which of the following inhaled general anesthetic agents has an MAC that exceeds normal atmospheric pressure?

  A. enflurane
  B. halothane
  C. isoflurane
  D. nitrous oxide
  E. sevoflurane
  D. nitrous oxide

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  Explanation

  Section: Pharmacology The MAC value for nitrous oxide is 105110 % (i.e., 510% greater than atmospheric pressure), meaning that for nitrous oxide to be used as the sole anesthetic agent, hyperbaric conditions would have to be used to deliver both the nitrous oxide for anesthesia and oxygen needed for life. The other commonly used inhaled agents, including the halogenated hydrocarbons enflurane, isoflurane, halothane, and sevoflurane (choices A, B, C, and E) have MAC values of 6% or less. Although these data might be interpreted to imply that nitrous oxide is not useful in anesthesia, nitrous oxide is very valuable because of its very low toxicity and additive effects with more potent (but more toxic) agents. It is therefore commonly combined with other inhaled agents, especially the halogenated hydrocarbons. For example, 0.5% MAC of nitrous oxide plus 0.5% MAC of isoflurane delivers a full anesthetic concentration of gases to the lungs but requires <60% of the inspired air partial pressure, so oxygen can utilize the other 40%.

• Question 469:

  A 37-year-old man was immunologically competent but presented to his physician with malaise, fever, lethargy, and back pain. Examination also revealed vertebral osteomyelitis, splenomegaly, and hepatomegaly. He was a PPD-negative farm worker who consumed unpasteurized goat milk. Which of the organisms listed below is the mostlikely etiological agent?

  A. Actinomyces israelii
  B. Aspergillus fumigates
  C. Brucella melitensis
  D. Candida albicans
  E. Mycobacterium tuberculosis
  C. Brucella melitensis

• Question 470:

  Aspecimen of thick, bloody sputum from a hospitalized 80-year-old patient with chronic obstructive pulmonary disease is sent for laboratory analyses. The tests yield heavy growth of a lactose-positive, nonmotile, gram-negative rod with a large gelatinous capsule. Which of the following is this bacterium most likely to be?

  B. Enterobacter aerogenes
  C. K. pneumonia
  D. P. aeroginosa
  E. Y. pseudotuberoulosis
  C. K. pneumonia

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  Explanation

  Section: Microbiology/Immunology

  K. pneumoniae (choice C) is present in the respiratory tract and feces of about 5% of normal individuals. It causes a small proportion (about 1%) of bacterial pneumonias. It is a gram-negative rod with a well-defined capsule that helps to resist phagocytosis by PMNs. It can produce extensive hemorrhagic necrotizing consolidation of the lung in debilitated patients and can cause nosocomial (hospital acquired) infections. Elderly patients with COPD are particularly at risk with this organism. Other enteric

  (choices A and B) may also produce pneumonia but do not have the well-defined capsule that

  K. pneumoniae possesses.

  P. aeruginosa (choice D) is a widely distributed gramnegative bacillus that can also be quite opportunistic for individuals with abnormal host defenses.

  P. aeruginosa produces pigments that produce bluegreen pus and is especially a problem with cystic fibrosis patients where the organism resides in a well-defined biofilm within the patient.

  Y. pseudotubeculosis (choice E) resides in

  domestic farm animals and is the source of human infections.