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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 651:

  A 9-month-old child is presented to the emergency room by his parents who report that he has been vomiting and has severe diarrhea. The episodes of vomiting began when the parents started feeding their child cow's milk. The infant exhibits signs of failing to thrive. Laboratory tests show elevated blood galactose, hypergalactosuria, metabolic acidosis, albuminuria, and hyperaminoaciduria. These clinical and laboratory findings are most consistent with which of the following disorders?

  A. alkaptonuria
  B. essential fructosuria
  C. hereditary galactosemia
  D. Menkes disease
  E. von Gierke disease
  C. hereditary galactosemia

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  Explanation

  Section: Biochemistry Severe hereditary galactosemia presents in the first few months of life with symptoms that include poor feeding and associated weight loss, vomiting, diarrhea, lethargy, and hypotonia. Clinical findings will include those presented in the case. The symptoms are aggravated by consumption of cow's milk and can be resolved provided proper diagnosis is made and treatment is started early. Alkaptonuria (choice A) results from the accumulation of homogentisic acid, a by-product of tyrosine catabolism, in the urine and tissues. Oxidation of homogentisate in the urine causes it to turn dark and in the tissues results in ochronosis, which refers to the ochre color of the deposits in connective tissue, bones, and other organs. Essential fructosuria (choice B) is a benign asymptomatic metabolic disorder manifesting with alimentary hyper- fructosemia and fructosuria. Menkes disease (choice D) results from a defect in intracellular copper transport and the symptoms of the disease are caused by loss of function of copper- dependent enzymes. Symptoms include abnormal (kinky) hair and pigmentation, cerebral degeneration, failure to thrive, and skin laxity. Symptoms of von Gierke disease (choice E) result from the excessive accumulation of glycogen in liver, kidney, and intestinal mucosa. Symptoms include growth retardation, hypoglycemia, heptomegaly, hyperlipidemia, lactic acidemia, and hyperuricemia.

• Question 652:

  Many proteins undergo modifications during and/or following translation. The ATPdependent polyubiquitination of proteins is a signal for which of the following events?

  A. addition of the oligosaccharide core to the N-linkage site adjacent to the ubiquitin
  B. recognition by the toxin of diphtheria
  C. recycling of the protein back to the endoplasmic reticulum (ER) from the cis-Golgi to allow proper folding
  D. targeting the protein for degradation in the proteosome
  E. translocation of the protein into the nucleus
  D. targeting the protein for degradation in the proteosome

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  Explanation

  Section: Biochemistry The ATP-dependent poly-ubiquitination of proteins is a signal that the protein is misfolded and thus, needs to be targeted by the proteosome for degradation. Mono-ubiquitination is known to initiate cell signaling by allowing other proteins that contain ubiquitin-binding domains to interact with the mono- ubiquitinated substrate. Mono-ubiquitination has also been associated with targeting of membrane proteins to the lysosome. None of the other examples (choices AC, E) are associated with addition of ubiquitin.

• Question 653:

  A drug being developed as an antiarrhythmic agent was studied in the laboratory using microelectrode techniques for measuring the transmembrane potential. The results of this study are shown in below figure. Which of the following standard antiarrhythmic agents does the new drug most resemble?

  

  A. adenosine
  B. ibutilide
  C. lidocaine
  D. procainamide
  E. propranolol
  B. ibutilide

• Question 654:

  Administration of which one of the following substances is likely to be most beneficial to this patient?

  A. gamma interferon
  B. interleukin-1
  C. plasmapheresis
  D. prednisone
  E. tumor necrosis factor (TNF)
  D. prednisone

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  Explanation

  Section: Microbiology/Immunology Prednisone alone or in combination with methotrexate and azathioprime is commonly used for the treatment of systemic lupus erythematosus. The cytokines TNF (choice E), interleukin-1 (choice B), or gamma interferon (choice A) have not been shown to be effective against SLE. Plasmapheresis (choice C) has been used for the treatment of Guillain-Barre syndrome and myasthenia gravis

• Question 655:

  The patient's viral infection can best be prevented by which of the following treatments?

  A. administration of human gamma globulin
  B. immunization with HBcAg
  C. immunization with the specific inactivated virus
  D. immunization with the specific subunit vaccine
  E. interferon
  C. immunization with the specific inactivated virus

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  Explanation

  Section: Microbiology/Immunology Hepatitis virus Ainfection can best be prevented by immunization with the intact inactivated virus. Interferons act only in the early phase of a viral infection, limiting the spread of virus, and are not recommended for long-term protection against viral diseases (choice E). Immunization with a subunit vaccine is used to prevent hepatitis virus B infection, not hepatitis virus Ainfection (choice D). Hepatitis B core antigen (HBcAg) is not effective for vaccination, even against the hepatitis B virus from which it is derived (choice B). Administration of human gamma globulin may provide only passive immunity, which will be of short duration, and thus it is not the best way to prevent hepatitis A viral infection (choice A).

• Question 656:

  A 19-year-old freshman football player collapses during preseason workouts on a very hot afternoon. He has no significant prior medical history other than a recent upper respiratory infection. He is taken to the emergency room but cannot be revived. An autopsy reveals an enlarged heart that, on microscopic examination, shows myocyte disarray. Which of the following was the most likely cause of death?

  A. alcoholism
  B. cardiac amyloidosis
  C. heat stroke
  D. hypertrophic cardiomyopathy
  E. viral myocarditis
  D. hypertrophic cardiomyopathy

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  Explanation

  Section: Pathology and Path physiology Hypertrophic cardiomyopathy often presents with dyspnea and chest pain. Arrhythmias are a common problem in these individuals and sudden death, particularly after very strenuous exercise, can occur. There is good evidence that at least half of these cases are familial--probably autosomal dominant. Adistinctive microscopic finding is myocyte disarray. Alcoholism (choice A) can produce dilated cardiomyopathy which does not show myocyte disarray. Cardiac amyloidosis (choice B) would not be expected in a young adult with no history of chronic disease and would show interstitial hyaline deposits of amyloid microscopically. Heat stroke (choice C) is marked by a lifethreatening increase in core temperature and would not produce the changes described above. Viral myocarditis (choice E) can produce an enlarged flabby heart but does not have myocyte disarray

• Question 657:

  An emaciated prisoner in a New York prison began coughing up sputum streaked with blood. Examination of the sputum revealed the presence of acid-fast bacilli. Which of the following would be least helpful in establishing a diagnosis of tuberculosis?

  A. agglutination test for antibodies
  B. niacin production test
  C. nitrate reduction test
  D. rough, non-pigmented colonies
  E. slow growth on Lowenstein-Jensen medium
  A. agglutination test for antibodies

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  Explanation

  Section: Microbiology/Immunology Molecular amplification techniques provide fast diagnostic results when they are compared to traditional cultural procedures for M. Tuberculosis, which often has to be incubated for several weeks. The rod- shaped, aerobic bacteria do not stain readily, but once stained they resist decoloration by acid or alcohol (hence, acid-fast bacilli). Cellular immunity is most important in how a host controls the growth and spread of the tubercle bacteria. There is evidence that mononuclear phagocytes ingest the organisms and even destroy them. The tuberculin skin test is based on cellular immune mechanisms, with the PPD causing a positive skin reaction in a person who has encountered the mycobacterium in the past. As a result, humoral immunity reactions are of little use diagnostically (choice A). Other mycobacteria are capable of accumulating niacin (choice B) and reducing nitrate (choice C). The development of rough, non-pigmented colonies on media (choice D) and slow growth on L-J medium (choice E) are also not specific for M. tuberculosis since slow growth (weeks) and colony morphology (rough, no pigment) are characteristic of other species of Mycobacterium.

• Question 658:

  While on a wilderness vacation, a banker develops extensive skin lesions noted for their varying degrees of erythema, edema, and vesiculations. His physician tells him that these lesions are due to delayed type hypersensitivity. If this is actually the case, which of the following statements is accurate?

  A. Delayed-type hypersensitivity can be transferred passively to volunteers by sensitized lymphocytes.
  B. Delayed-type hypersensitivity is suppressed by antihistaminic drugs.
  C. This allergy does not cause tissue damage.
  D. This allergy is due to IgE absorbed on mast cells.
  E. This type of allergy usually occurs after inhalation of grass pollens.
  A. Delayed-type hypersensitivity can be transferred passively to volunteers by sensitized lymphocytes.

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  Explanation

  Section: Microbiology/Immunology Delayed-type hypersensitivity is mediated by helper CD4 lymphocytes, not antibody (choice D). Thus it can only be transferred by sensitized CD4 helper lymphocytes. Sensitization occurs via such substances as poison ivy, poison oak, some cosmetics, topically applied sulfonamides, or other drugs and simple chemicals, such as nickel, formaldehyde, and others, but not inhalation of grass pollen (choice E). Part of the tissue destruction seen in tuberculosis is due to delayed-type hypersensitivity, so this type of hypersensitivity reaction may lead to tissue damage (choice C). Administration of antihistaminic drugs, epinephrine, or cromolyn sodium is used for the treatment of anaphylactic reactions resulting from type I hypersensitivity, but not delayed-type hypersensitivity (choice B).

• Question 659:

  Which of the following agents used in general anesthesia is least likely to cause postoperative nausea and vomiting?

  A. enflurane
  B. etomidate
  C. isoflurane
  D. morphine
  E. propofol
  E. propofol

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  Explanation

  Section: Pharmacology Propofol may reduce blood pressure through vasodilating and negative inotropic effects on the heart, but is notable for rapid recovery and its freedom from postoperative nausea and vomiting. For this reason, it is extremely popular for day surgery. None of the other anesthetics (choices AD) are free from causing postoperative nausea.

• Question 660:

  Which of the following is true with respect to the actions of the mineralocorticoids?

  A. decrease carbohydrate metabolism
  B. increase appearance of the secondary sex characteristics
  C. increase synthesis of androgens
  D. regulate aldosterone secretion
  E. regulate sodium retention by the kidneys
  E. regulate sodium retention by the kidneys

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  Explanation

  Section: Biochemistry The principal mineralocorticod, produced by the zona glomerulosa cells of the adrenals, is aldosterone. Synthesis of aldosterone is primarily controlled by the rennin-angiotensin system and is thus involved in control of blood pressure. Aldosterone causes sodium reabsorption by the kidneys, which in turn regulate water balance, which leads to increases in blood pressure by increasing fluid volume. Aldosterone action does not directly lead to decreased carbohydrate metabolism (choice A), is not a sex characteristic determining steroid hormone (choice B), does not result in increased synthesis of androgens (choice C), and does not regulate its own secretion (choice D).