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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 241:

  Infants exhibiting profound metabolic ketoacidosis, muscular hypotonia, developmental retardation, and who have very large accumulations of methylmalonic acid in their blood and urine suffer from a disorder known as methylmalonic acidemia. This disorder results from a defect in which of the following enzymes?

  A. alpha-keto acid dehydrogenase
  B. homogentisic acid oxidase
  C. methylmalonyl-CoA mutase
  D. phenylalanine hydroxylase
  E. tyrosine aminotransferase
  C. methylmalonyl-CoA mutase

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  Explanation

  Section: Biochemistry Defects in methylmalonyl-CoA mutase activity comprise four distinct genotypes whose clinical symptoms are remarkably similar. Characteristic findings in methylmalonyl-CoA mutase deficiency include failure to thrive leading to developmental abnormalities, recurrent vomiting, respiratory distress, hepatomegaly, and muscular hypotonia. In addition, patients have severely elevated levels of methylmalonic acid in the blood and urine. Unaffected individuals have near undetectable levels of methylmalonate in their plasma, whereas, affected individuals have been found to have levels ranging from 3 to 40 mg/dL in their blood. Deficiency in alpha-ketoacid dehydrogenase (choice A) results in MSUD, so named because of the characteristic odor of the urine in afflicted individuals. Mental retardation in the MSUD is extensive. Deficiency in homogentisic acid oxidase (choice B) results in alkaptonuria. Alkaptonuria results from the accumulation of homogentisic acid, a byproduct 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. Deficiency in phenylalanine hydroxylase (choice D) results in PKU which results in severe mental retardation if not detected and treated properly. Deficiency in tyrosine aminotransferase (choice E) results in eye, skin, and neurologic symptomology. The neurologic symptoms are similar to those seen in PKU.

• Question 242:

  A 4-year-old patient is presented in the pediatric clinic with microcytic anemia. An analysis of his blood by nondenaturing electrophoresis reveals the following composition of hemoglobin isoforms:

  HbF = 75%, HbA = 23%, HbA2 = 2%, and HbS = 0%. Using these data, is it possible to determine that the infant is most likely homozygous for which of the following?

  A. complete deletion of the alpha-globin locus
  B. complete deletion of the beta-globin locus
  C. mutation in the promoter of the betaglobin genes
  D. nonsense mutation in the alpha-globin genes
  E. nonsense mutation in the beta-globin genes
  C. mutation in the promoter of the betaglobin genes

• Question 243:

  Ajudge has a fungal infection known as "Tinea cruris." Which one of the following statements concerning this infection is accurate?

  A. It cannot be transmitted from person to person.
  B. It involves a fungus that produces aflatoxins.
  C. It is associated with athlete's foot.
  D. It is caused by a fungus that forms tuberculate macroconidia.
  E. It occurs primarily in immunocompromised individuals.
  C. It is associated with athlete's foot.

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  Explanation

  Section: Microbiology/Immunology Tinea cruris or as commonly known "Jock itch" is typically found in young men, especially in macerated or occluded areas. It is also associated with tinea pedis (athlete's foot) and the causative agent (T. rubrum or T. mentagrophytes) may be easily seen in skin scrappings. Tinea cruris occurs in ealthy and immunosuppressed persons (choice E). Histoplasma capsulatum, the causative agent of histoplamosis, forms septate hyphae with tuberculate macroconidia (choice D). In contrast to other fungal infections, tinea cruris is transmitted from person to person (choice A). Aspergillus flavus, not T. rubrum or T. mentagrophytes, produces aflatoxins, which can cause liver damage (choice B).

• Question 244:

  Aphysician is treating a patient whom he suspects of factitious disorder. If the physician learns that the patient would have been incarcerated for a criminal offense if he had not been hospitalized, which of the following diagnoses becomes most likely?

  A. antisocial personality disorder
  B. borderline personality disorder
  C. malingering
  D. Munchausen syndrome
  E. sick-role addiction
  C. malingering

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  Explanation

  Section: Behavioral Science and Biostatics When there is an external incentive for causing the symptom or injury, malingering is more likely than factitious disorder (Munchausen syndrome), the motivation for which is often quite unclear. Although there may be concomitant personality disorders, malingering is not diagnostic of any specific personality disorder. Choices A, B, D, and E are less likely because there seems to be an obvious and conscious motivation in this case.

• Question 245:

  Calcium- and aluminum-containing antacids should be avoided by patients taking which of the following antimicrobial drugs?

  A. amoxicillin
  B. erythromycin
  C. isoniazid
  D. minocycline
  E. rifampin
  D. minocycline

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  Explanation

  Section: Pharmacology Tetracyclines such as minocycline chelate cations, such as aluminum and calcium, and may precipitate in the gut, greatly reducing antibiotic bioavailability. The other drugs listed (choices AC, E) have no significant interactions with antacids.

• Question 246:

  Viruses have various ways of entering the human body and producing disease. Which of the following descriptions accurately describe the route and mechanism for the virus indicated?

  A. Coronaviruses enter the gastrointestinal tract through the mouth and move into the stomach, where they proliferate within mucosal cells to produce peptic ulcers.
  B. Enteroviruses enter through the mouth, replicate in the pharynx and bowel, and move via the blood to distant target organs (central nervous system).
  C. HIV is directly injected into the blood stream by insects requiring blood for egg development.
  D. Influenza viruses enter through the respiratory tract, replicate within lymphocytes in the lung, and move via the lymphatic vessels to joints and the central nervous system to produce muscle aches, stiff joints, and fever.
  E. Mumps virus enters through abraded skin in the genital area and moves into the testicles of males to produce swelling and sterility.
  B. Enteroviruses enter through the mouth, replicate in the pharynx and bowel, and move via the blood to distant target organs (central nervous system).

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  Explanation

  Section: Microbiology/Immunology It is important that physicians have a good understanding of how pathogens, including viruses, enter into the human host and know what symptoms may be caused by the microorganism. Such knowledge and interpretations may have significant value in making a meaningful differential diagnosis. Enteroviruses do enter through the mouth, replicate in the pharynx and bowel before using the blood system (viremia) to access final target organs (choice B). Coronaviruses (choice A) are inhaled or ingested (small number of strains), but do not produce peptic ulcers. Insects have been shown to not be important for HIV transmission (choice C). Influenza viruses (choice D) do not replicate in lung lymphocytes. Mumps virus (choice E) is most often inhaled from respiratory secretions from infected individuals who make large numbers of the viruses in the upper respiratory tract during the prodromal period.

• Question 247:

  A 53-year-old male patient has developed an abnormal hypersensitivity in the face area. He complains that he feels pain when he drives with the window down and from the wind blowing on his face. The pain is exacerbated by direct touch to the left side of his lower jaw. The patient is diagnosed with trigeminal neuralgia (tic douloureux). Where do pain fibers from the left side of his mandible project to?

  A. mesencephalic trigeminal nucleus
  B. motor trigeminal nucleus
  C. principal sensory trigeminal nucleus
  D. spinal (descending) trigeminal nucleus
  E. trigeminal (gasserian) ganglion
  D. spinal (descending) trigeminal nucleus

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  Explanation

  Section: Anatomy Fibers from pain and temperature receptors in the face area all project to the spinal (descending) trigeminal nucleus. The mesencephalic trigeminal nucleus (choice A) contains the perikarya of the proprioceptive fibers of the muscles of mastication. The motor trigeminal nucleus (choice B) receives innervations from the mesencephalic trigeminal nucleus. The principal sensory trigeminal nucleus (choice C) receives somatosensory fibers from the face, gums, teeth, and anterior two-thirds of the tongue. The trigeminal (gasserian) ganglion (choice E) contains the sensory perikarya of all three subdivisions of the trigeminal nerve.

• Question 248:

  Aurease-positive, spiral-shaped gram-negative rod is found in gastric washings from a 29-year-old stressed-out medical student with gastritis. Which of the following is most likely to be this organism?

  A. Campylobacter jejuni
  B. Helicobacter pylori
  C. Proteus mirabilis
  D. Providencia rettgeri
  E. Vibrio cholerae
  B. Helicobacter pylori

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  Explanation

  Section: Microbiology/Immunology

  H. pylori is a spiral-shaped gram-negative motile rod that is associated with antral gastritis and duodenal (peptic) ulcer disease. It is oxidase-positive, catalase-positive, and a strong producer of urease. Antimicrobial therapy that clears

  H. pylori results in improvement of gastritis and duodenal ulcer disease. Hence, choice B is the correct answer.

  C. jejuni (choice A) infections are widespread and usually cause abdominal pain and profuse bloody diarrhea, not gastritis and ulcer manifestations. Ureasepositive

  P. mirabilis (choice C) and other Proteus species produce infections in humans when the bacteria leave the intestinal tract, being very significant in urinary tract and bacteremia infections.

  P. retteri (choice D) is a member of the normal intestinal flora, often resistant to antimicrobial therapy, and is seen in urinary tract infections.

  V. cholerae is a comma-shaped curved rod and is widely distributed in marine and surface waters in nature. It is oxidase positive and many species are salt tolerant.

  V. cholerae (choice

  E) produces a heat-labile enterotoxin whose action increases cAMP in intestinal cells, resulting in prolonged hypersecretion of water and electrolytes.

• Question 249:

  Referring to following figure what is the most likely source of this epithelium?

  

  A. gall bladder
  B. salivary duct
  C. thick skin
  D. trachea
  E. urinary bladder
  E. urinary bladder

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  Explanation

  Section: Anatomy The appearance of this epithelium reveals it to be transitional epithelium. It is stratified with a scalloped surface outline. The cells at the bottom layer are cuboidal in appearance and stained darkly; cells in the intermediate layer are polygonal. Cells at the surface of the epithelium are pale-stained, rounded, and large. Transitional epithelium is characteristic of organs of the urinary system, such as the urinary bladder. The gall bladder (choice A) is characterized by simple columnar epithelium. The epithelium of the salivary gland (choice B) is simple cuboidal epithelium. Thick skin (choice C) is made up of stratified squamous epithelium and the trachea (choice D) is lined with pseudostratified columnar epithelium.

• Question 250:

  Which of the following drugs inhibit osteoclastic activity more than osteoblastic activity and have been beneficial in osteoporosis, hypercalcemia of malignancy, and Paget disease?

  A. alendronate
  B. calcitonin
  C. fluoride
  D. parathyroid hormone
  E. vitamin D
  A. alendronate

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  Explanation

  Section: Pharmacology Alendronate is a bisphosphonate, the class of drugs that has proved most effective in the treatment and prevention of osteoporosis. This class of drug is also useful in the management of hypercalcemia and certain neoplasms. Calcitonin (choice B) has been shown to slow bone loss in postmenopausal women and to treat postoperative pancreatitis. Fluoride (choice C) is used primarily in the prevention of dental caries. Parathyroid hormone (choice D), marketed as Forteo, is currently used in the clinic to treat osteoporosis. Vitamin D (choice E) has shown benefit in increasing bone deposition in osteoporotic patients. However, choices BE do not have clinical use in the treatment of malignancy or Paget disease.