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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 431:

  Your patient reports that she is suffering from dizziness and persistent feelings that she is going to die. You inquire as to whether she has been using any illicit substances lately to which she indicates that she has not. She indicates that she has been curtailing going outside, especially in open spaces, because she is afraid that she might have one of those attacks and become immobilized, which of the following is the most likely diagnosis?

  A. acute anxiety attack
  B. agoraphobia with panic attack
  C. agoraphobia without panic attack
  D. anxiety secondary to a medical condition
  E. cocaine withdrawal
  B. agoraphobia with panic attack

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  Explanation

  Section: Behavioral Science and Biostatics The symptoms of panic associated with fear and avoidance of being in places where escape may be difficult or embarrassing call for the consideration of agoraphobia with panic attack. Choices A, C, D, and E are not associated with the phenomenon described here.

• Question 432:

  Apatient with a developing astroglioma in the primary auditory cortex (Brodmann's area 41) complains of hearing unusual noises. What is the main source of subcortical input to the primary auditory cortex?

  A. inferior colliculus
  B. lateral geniculate nucleus
  C. medial geniculate nucleus
  D. superior colliculus
  E. superior olivary nucleus
  C. medial geniculate nucleus

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  Explanation

  Section: Anatomy The main subcortical afferents to the primary auditory cortex (Brodmann's area 41) arise from the medial geniculate nucleus of the thalamus. The inferior colliculus (choice A) is a midbrain auditory relay nucleus, which sends projections to the medial geniculate nucleus. The lateral geniculate nucleus (choice B) is a visual relay nucleus in the thalamus and it projects to the primary visual cortex (Brodmann's area 17). The superior colliculus (choice D) is a midbrain visual relay nucleus, which provides fibers to the tectospinal tract. The superior olivary nucleus (choice E) is a medullary auditory relay nucleus, which sends projections to the cochlea.

• Question 433:

  With increasing age, it is quite common for adults to have very low sensitivity for highpitch sounds. Which of the following physiologic responses occur as the pitch of a sound is increased at constant sound pressure?

  A. a greater number of hair cells become activated
  B. the amplitude of maximal basilar membrane displacement increases
  C. the frequency of action potentials in auditory nerve fibers increases
  D. the location of maximal basilar membrane displacement moves toward the cochlear base
  E. units in the auditory nerve become responsive to a wider range of sound frequencies
  D. the location of maximal basilar membrane displacement moves toward the cochlear base

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  Explanation

  Section: Physiology The primary change in the cochlea due to an increase in the frequency of a sound wave is a change in the position of maximal displacement of the basilar membrane. A low-pitched sound produces the greatest displacement toward the apex of the cochlea, which results in activation of hair cells at that location. As pitch increases, the position of greatest displacement moves closer to the base of the cochlea. Increases in the number of hair cells that are activated (choice A), increased amplitude of basilar membrane displacement (choice B), increases in the frequency of discharge of units in the auditory nerve fibers (choice C), and an increase in range of frequencies to which such units respond (choice E), are all more likely to be observed in response to increases in the intensity of the sound stimulus (sound pressure) rather than to increases in pitch.

• Question 434:

  Which of the following agents would cause the greatest increase in heart rate in a functioning transplanted heart?

  A. amphetamine
  B. cocaine
  C. ephedrine
  D. isoproterenol
  E. terbutaline
  D. isoproterenol

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  Explanation

  Section: Pharmacology The transplanted heart lacks functional innervation for at least 2 years after surgery, and possibly longer. Because the nerves are cut in the procedure, nerve endings degenerate (Wallerian degeneration) and transmitter stores are lost. Adrenoreceptors on the denervated myocardial cells are normal or even increased in sensitivity, so responses to direct-acting betaadrenoreceptor agonists such as isoproterenol are retained. Indirectly acting sympathomimetics, on the other hand, are relatively ineffective (choices A, B, and C) because they act through the release or amplification of endogenous

• Question 435:

  Robert is 9 years old and has been suspended from school for failing to control his disruptive behavior in the classroom. His parents take him to a psychologist and report that he has been hyperactive and impulsive since the age of 5. Which of the following is the drug of choice for the treatment of this child?

  A. buspirone
  B. clomipramine
  C. dextroamphetamine
  D. nefazodone
  E. paroxetine
  C. dextroamphetamine

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  Explanation

  Section: Behavioral Science and Biostatics Dextroamphetamine and methylphenidate are effective in treating ADHD. Paroxetine (choice E) and nefazodone (choice D) are antidepressants, clomipramine (choice B) is a tricyclic that has special use in obsessive-compulsive disorders, and buspirone (choice A) is an antianxiety agent.

• Question 436:

  The tolerance of facultative anaerobic bacteria, many of which may be normal flora and opportunistic, to superoxide and hydrogen peroxide is due to which of the following?

  A. absence of catalase
  B. cytochrome oxidase
  C. inability to form oxygen
  D. presence of peroxidase
  E. presence of superoxide dismutase and catalase
  E. presence of superoxide dismutase and catalase

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  Explanation

  Section: Microbiology/Immunology Superoxide, which is bactericidal, is generated during electron transport and in the autoxidation of hydroquinones, leukoflavins, ferredoxins, and flavoproteins. Superoxide dismutase, however, forms oxygen and hydrogen peroxide from superoxide radicals. When catalase is present, it destroys the bactericidal hydrogen peroxide, because catalase hydrolyzes hydrogen peroxide to water and oxygen. Thus, the presence of both enzymes allows aerobes, facultative anaerobes, andaerotolerant microbes to survive when they grow in the presence of oxygen. Obligate anaerobic bacteria generally lack superoxide dismutase or catalase or both and thus cannot grow in the presence of oxygen. Oxidase is an enzyme that oxidizes a substrate by the addition of oxygen or the removal of hydrogen. It cannot oxidize the bactericidal radical superoxide (choice B). Catalase is an enzyme that converts the bactericidal substance hydrogen peroxide to water and oxygen. It has no effect on the microbiocidal action of superoxide. Bacteria that lack catalase are likely to be suppressed by hydrogen peroxide (choice A). Peroxidase is an enzyme which catalyzes the dehydrogenation of a substrate in the presence of hydrogen peroxide, which acts as hydrogen acceptor and becomes converted into two molecules of water. Thus, possession of peroxidase in a given microbe will allow it to grow in the presence of hydrogen peroxide, but not in the presence of superoxide (choice D). There are no known pathogenic bacteria that cannot form oxygen (choice C).

• Question 437:

  A 53-year-old alcoholic man had been experiencing acute episodes of nausea and eventually jaundice. Avirus was detected from him, which had a circular molecule of DNA having the structure diagrammed in below figure. Which virus listed below has this type of genome?

  

  A. EBV
  B. hepatitis A virus
  C. hepatitis B virus
  D. JC virus
  E. papillomavirus
  C. hepatitis B virus

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  Explanation

  Section: Microbiology/Immunology The genome of hepatitis B virus is composed of a circular, double-stranded DNA. It has a negative strand of 3200 nucleotides and another positive, incomplete strand of 17002600 nucleotides. Papilloma (choice E) and polyomaviruses belong to the family of Papoviridae, which lacks envelopes and have a double- stranded, circular DNA genome. Both strands are complete and thus differ from the genome of hepatitis B virus that has one incomplete positive strand. Hepatitis Avirus (choice B), the cause of infectious hepatitis, has a linear single-stranded RNA genome, and as such it is a member of the enteroviruses. EBV (choice A) is a member of the herpesviruses and as such has a double-stranded, linear DNA genome. EBV has been associated with infectious mononucleosis, Burkitt lymphoma, and nasopharyngeal carcinoma. JC virus (choice D) is a member of the polyomaviruses.

• Question 438:

  A 27-year-old graduate student, upon her return from vacation in a tropical area with poor sanitary conditions, became ill. She experienced lower abdominal pain, colitis, tenesmus, flatulence, and bloody diarrhea caused by an RBCcontaining parasite. Which of the following represents the most likely diagnosis?

  A. acute amoebic dysentery
  B. malaria
  C. toxoplasmosis
  D. trichomoniasis
  E. trypanosomiasis
  A. acute amoebic dysentery

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  Explanation

  Section: Microbiology/Immunology The most likely diagnosis of the patient's illness is acute amoebic dysentery, because the classical symptomatology of acute amoebic dysentery is lower abdominal pain, cramping, colitis, tenesmus, flatulence, and bloody diarrhea. Malaria is characterized by periodic fever, fatal cerebral episodes, lysis of red blood cells, and nephritis due to immune complex formation, and is transmitted by mosquito bites (choice B). Trypanosomiasis is transmitted by bites of the tsetse fly and the trypanosomes initially cause chancres at the site of the bite. Then the trypanosomes reach the central nervous system, and the patient has lassitude, develops sleeping episodes, and tissue wasting or even death can occur (choice E). Trichomoniasis is limited to the vagina and is associated with a watery, foul smelling, abundant greenishgray discharge (choice D). Toxoplasmosis is a mild influenza-like disease with possible lymph node enlargement, and may be severe in immunocompromised patients. Congenital infections can damage the eyes or brain and may be fatal (choice C).

• Question 439:

  During a routine pediatric visit, you discover a large abdominal mass in a 2-year-old boy. The mass is surgically removed and is illustrated in below figure. What is the most likely diagnosis?

  

  A. abscess
  B. neuroblastoma
  C. renal cell carcinoma
  D. tuberculosis
  E. Wilms' tumor
  E. Wilms' tumor

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  Explanation

  Section: Pathology and Path physiology Figure shows a large tumor mass originating in the kidney. Wilms' tumor (or nephroblastoma) is the most common primary renal tumor of childhood and the second most common malignancy overall after lymphoma/ leukemia. It typically presents as a large abdominal mass discovered by a parent. An abscess (choice A) is a localized collection of pus that is not compatible with the solid mass shown here. A neuroblastoma (choice B) is most commonly primary in the adrenal and is unlikely to arise in the kidney. Renal cell carcinoma (choice C) is the most common primary renal tumor in adults, but is not expected in a child. Tuberculosis (choice D) would be identified by caseous necrosis, which has the appearance of amorphous crumbled cheese, not the solid appearance shown here.

• Question 440:

  Mumps virus accounts for 1015% of all cases of aseptic meningitis in the United States. Which of the following is most true of infection with mumps virus?

  A. It is apt to recur periodically in many affected persons.
  B. It is maintained in large canine reservoir.
  C. It is preventable by immunization.
  D. It usually produces severe systemic manifestations.
  E. It will always cause mumps orchitis in postpubertal males.
  C. It is preventable by immunization.

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  Explanation

  Section: Microbiology/Immunology Mumps is an acute contagious disease characterized by nonsupportive enlargement of one or both salivary glands. While it is estimated that one-third of all infections are subclinical, mumps in children is mild. In adults, complications include meningitis and orchitis. Immunity is permanent after a single infection or vaccination with the single antigenic type live-attenuated vaccine (choice C). As stated, it does not recur in infected individuals (choice A) and is a human pathogen, not canine (choice B). Meningitis and orchitis are the main complications, but severe systemic manifestations (choice D) do not occur. Orchitis (choice E) occur in 2050% of infected men.