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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 611:

  A 37-year-old man visiting a third world country drinks from a fecally contaminated source of water. During the next week he has gradually increasing fever, anorexia, myalgia, and headache. He subsequently develops a maculopapular rash on his abdomen and his fever increases to 104°F with abdominal pain and splenomegaly. During the third week, his condition rapidly deteriorates with intestinal bleeding, shock, and death. An autopsy reveals ulcerations overlying the Peyer patches of the small intestine, one of which is perforated. Which of the following is the most likely diagnosis?

  A. amebiasis
  B. cholera
  C. cryptosporidiosis
  D. giardiasis
  E. typhoid fever
  E. typhoid fever

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  Explanation

  Section: Pathology and Path physiology This is a case of typhoid fever. During the prodromal stage there is a gradual, step-like increase in fever with malaise, anorexia, myalgia, and headache. By the second week, the fever usually plateaus and the patient is very sick. There may be constipation or diarrhea with abdominal pain and distention, weakness, and a maculopapular rash (rose spots), particularly on the abdomen. If there are no complications, the patient may gradually improve over the next 2 weeks. One of the classic pathologic findings in typhoid is intestinal ulcerations over hyperplastic and necrotic Peyer patches. These may perforate and hemorrhage, as in this patient. Amebiasis (choice A) can produce a range of symptoms from being subclinical to producing fulminant dysentery. However, it involves the large bowel rather than the small intestine and produces flask-shaped ulcers separated by areas of normal bowel. Cholera (choice B) does not actually invade the intestinal epithelium and therefore causes insignificant microscopic changes and no ulceration. Cryptosporidiosis (choice C) causes a severe, chronic diarrhea in AIDS patients. The protozoa attach to the brush border of the intestinal epithelial cells but do not cause ulceration. Giardia (choice D) attaches to duodenal epithelial cells, but does not invade those cells and does not cause ulceration.

• Question 612:

  Which of the following immunological assays employs separation of proteins by electrophoresis in polyacrylamide gels and subsequent transfer to nitrocellulose, followed by a reaction with antibodies?

  A. complement fixation
  B. enzyme-linked immunosorbent assay (ELISA)
  C. immunoelectrophoresis
  D. indirect fluorescent antibody test
  E. Western blot
  E. Western blot

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  Explanation

  Section: Microbiology/Immunology In Western blots proteins are separated on the basis of molecular weight in polyacrylamide gels by electrophoresis. Then, they are transferred electrophoretically to nitrocellulose membranes and reacted with antibodies and substrate for detection. The Western blot assay is used as a confirmatory test in the determination of the HIV status of an individual. Electrophoretic transfer of proteins or antigens to nitrocellulose membranes followed by antibody reaction is not a part of complement fixation (choice A), indirect fluorescent antibody test (choice D), ELISA (choice B), or immunoelectrophoresis (choice C).

• Question 613:

  Clinical laboratory results indicate that an AIDS patient in a county hospital in San Antonio is infected with an acid-fast bacillus. Which of the following bacteria is this organism most likely to be?

  A. M. avium-intracellulare
  B. M. fortuitum-chelonei
  C. M. marinum
  D. M. scrofulaceum
  E. M. tuberculosis
  A. M. avium-intracellulare

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  Explanation

  Section: Microbiology/Immunology

  M. avium-intracelluaire (choice A) is often called the MAC or MAI (M. avium-intracellulaire) complex. They are ubiquitous in the environment (water, soil, food, animals). MAC organisms seldom cause disease in immunocompetent humans but are efficient opportunists in those immunocompromised, especially in AIDS patients in the United States. In fact, it is one of the most common opportunistic infections of bacterial origin in that situation. Environmental exposure can lead to MAC colonization of either the

  respiratory or GI tract. A transient bacteremia is usually followed by tissue invasion. Any organ may be affected. A wide variety of clinical presentations may occur, but patients often present with nonspecific symptoms of fever, night sweats, abdominal pain, and diarrhea and weight loss. Diagnosis is made by culturing the organism. Treatment may involve multiple drugs and last from 6 months to 1 year or even for life. M. fortuitum-cheloni (choice B) are soil/water saprophytes and may cause superficial or

  systemic disease only rarely.

  M. marinum (choice C) occurs in water and may cause skin lesions in humans. Infections are relatively rare.M. scrofulaceum (choice D) is also found in water and may be a rare saprophyte in adults with chronic lung disease.

  M. tuberculosis (choice E) causes tuberculosis in humans and is a very important human pathogen. Host to host transmission is the usual method observed.

• Question 614:

  A 28-year-old woman has an ultrasound examination during the second trimester of her third pregnancy. She is found to have greatly decreased amniotic fluid but the fetus appears to be the appropriate size for the gestational age. Her two previous pregnancies produced normal-term infants. The family history is otherwise unremarkable. Which of the following conditions most likely explains these findings?

  A. bilateral cystic renal dysplasia
  B. bronchopulmonary dysplasia
  C. hypoplasia of the lungs
  D. Klinefelter syndrome
  E. placenta previa
  A. bilateral cystic renal dysplasia

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  Explanation

  Section: Pathology and Path physiology Most of the amniotic fluid is derived from the urine of the fetus. In a fetus with bilateral cystic renal dysplasia, little or no urine is being produced, which leads to the oligohydramnios seen in this case. Bronchopulmonary dysplasia (choice B) may be seen in babies who have respiratory distress syndrome and are given oxygen therapy. Hypoplasia of the lungs (choice C) is a consequence of oligohydramnios because inhalation of amniotic fluid by the fetus is necessary for normal lung development. Klinefelter syndrome (choice D) occurs in males with XXY karyotype and does not explain the findings in this case. Placenta previa (choice E) occurs when there is abnormally low implantation of the fertilized ovum in the uterus such that it overlies the internal os.

• Question 615:

  A patient with newly diagnosed schizophrenia is given chlorpromazine. It is a drug that has amongst other effects moderate anticholinergic activity. As a consequence, which of the following is an expected side effect of this medication?

  A. bradycardia
  B. decreased GI sphincter tone
  C. dry mouth
  D. emptying of urinary bladder
  E. increased GI motility
  C. dry mouth

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  Explanation

  Section: Physiology Chlorpromazine is an antipsychotic drug with anticholinergic action (i.e., it inhibits the effects of parasympathetic stimulation). Parasympathetic stimulation causes copious secretion by nasal, lacrimal, and parotid glands. Consequently, parasympathetic blockade is leading to a dry mouth. Parasympathetic fibers slow the heart rate and anticholinergic drugs cause tachycardia rather than bradycardia (choice A). Functions of the parasympathetic nervous system include decreasing GI sphincter tone, increasing GI motility, and increasing emptying of the rectum and urinary bladder. Parasympathetic blockade would thus cause increased GI sphincter tone (choice B), urinary retention (choice D), and decreased GI motility (choice E) leading to constipation.

• Question 616:

  A patient has completed a course of cancer chemotherapy and now has severe anemia, neutropenia, and thrombocytopenia. If only one intervention is possible, which of the following is the most appropriate therapy?

  A. epoetin (erythropoietin)
  B. filgrastim (G-CSF)
  C. growth hormone
  D. sargramostim (GM-CSF)
  E. testosterone
  D. sargramostim (GM-CSF)

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  Explanation

  Section: Pharmacology Apatient who is anemic, neutropenic, and thrombocytopenic requires stimulation of all three major cell lines in the bone marrow. The only drug currently available that accomplishes this broad-spectrum stimulant effect is sargramostim (granulocytemacrophage colony stimulating factor [GM-CSF]). Epoetin (choice A) is a more selective stimulant of erythrocyte production and is useful in simple anemia. Filgrastim (choice B) is a somewhat selective stimulant of leukocyte production and has much less effect on erythrocytes and platelet production than sargramostim. Growth hormone (choice C) and testosterone (choice E) have both been tried in the treatment of anemia with negligible success.

• Question 617:

  Apatient on intensive care is ventilated with a frequency of 12 per minute and a tidal volume of 0.6 L. His arterial pH increases to >7.5. What is the most reasonable action to correct this respiratory alkalosis?

  A. decrease dead space
  B. decrease tidal volume
  C. increase minute ventilation
  D. increase oxygen fraction
  E. use positive end-expiratory pressure (PEEP)
  B. decrease tidal volume

• Question 618:

  Which of the following drugs acts primarily on an intracellular receptor?

  A. abciximab
  B. dexamethasone
  C. insulin
  D. propranolol
  E. salmeterol
  B. dexamethasone

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  Explanation

  Section: Pharmacology Dexamethasone, a corticosteroid, combines with a cytoplasmic steroid receptor, enters the nucleus, and alters gene expression. The other agents listed (choices A, CE) act on membrane receptors.

• Question 619:

  A 54-year-old insulin-dependent diabetic notes that her insulin requirements have gone up dramatically in the past year (from 50 U to nearly 200 U of recombinant human insulin) and her blood glucose is still poorly controlled. A possible explanation for the worsening of her diabetes includes which of the following?

  A. a high titer of anti-insulin antibodies
  B. an improved diet
  C. an improved exercise program
  D. progression of macrovascular disease
  E. weight loss
  A. a high titer of anti-insulin antibodies

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  Explanation

  Section: Physiology The patient clearly has an increase in her state of insulin resistance. Given the magnitude of her increased insulin requirements, she most likely developed a high titer of antiinsulin antibodies that are preventing the injected insulin from lowering blood glucose effectively. Agood choice of lifestyle, including an improved diet (choice B), an improved exercise program (choice C), or weight loss (choice E) each are shown to beneficially affect her insulin requirements. Progression of macrovascular disease (choice D) is largely irrelevant to her insulin requirements, except to the extent that it might decrease her ability to exercise.

• Question 620:

  Which of the following is the only local anesthetic that causes vasoconstriction and may result in an increase in blood pressure?

  A. bupivacaine
  B. cocaine
  C. etidocaine
  D. lidocaine
  E. tetracaine
  B. cocaine

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  Explanation

  Section: Pharmacology Cocaine is the only local anesthetic that causes sympathomimetic effects (by inhibiting reuptake of norepinephrine into sympathetic nerve endings); it therefore regularly causes local vasoconstriction and may increase blood pressure. All the other local anesthetics (choices A, CE) cause vasodilation and lower blood pressure.