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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 711:

  A 25-year-old female executive presents with edema, hematuria, proteinuria, and decreased urination indicative of glomerulonephritis. Three weeks ago she had impetigo on her chest which resolved without treatment. Which of the following organisms is the most likely cause of her renal disease?

  B. S. aureus
  C. S. saprophyticus
  D. Streptococcus agalactiae
  E. S. pyogenes
  E. S. pyogenes

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  Explanation

  Section: Microbiology/Immunology Acute glomerulonephritis appears as a complication after pharyngeal or skin infections with Group A S. pyogenes. Patients with poststreptococcal glomerulonephritis develop high levels of antibodies against streptolysin O of S. pyogenes. While S. aureus (choice B) may also cause impetigo, it is seldom, if ever, found to be the cause of postinfectious glomerulonephritis. None of the other organisms (choices A, C, or D) have been identified with this postinfectious complication either.

• Question 712:

  The mechanism of action for the therapeutic effect of saquinavir involves which of the following?

  A. incorporation into RNA
  B. inhibition of thymidylate synthase
  C. inhibition of viral DNA polymerase
  D. inhibition of viral protease
  E. inhibition of viral reverse transcriptase
  D. inhibition of viral protease

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  Explanation

  Section: Pharmacology Saquinavir is an HIV protease inhibitor. Viral protease cleavage of a polyprotein is necessary for production of the viral coat protein. The HIV protease inhibitors include ritonavir, indinavir, nelfinavir, and amprenavir in addition to saquinavir. Currently available HIV protease inhibitors are not sufficient for monotherapy because of the rapid emergence of resistance due to mutations in the viral protease. Combination therapy using saquinavir with two reverse transcriptase inhibitors such as zidovudine, lamivudine, abacavir, or didanosine is currently effective in reducing viral load. Incorporation into RNA (choice A) is part of the mechanism of action for the antitumor agent 5-fluorouracil (5-FU). Inhibition of thymidylate synthase (choice B) is another part of the mechanism of action for the antitumor agent 5-FU. 5-FU is converted to 5- FdUMP, a potent inhibitor of thymidylate synthase. Inhibition of viral DNA polymerase (choice C) is a mechanism of action for the antiherpesvirus nucleosides acyclovir,

  valacyclovir, famciclovir, and ganciclovir. These agents are metabolized to their nucleotide triphosphate forms, which competitively inhibit the herpesvirus polymerase. Acyclovir and valacyclovir also cause DNA chain termination. Inhibition of viral reverse transcriptase (choice E) is a property of the nucleosides zidovudine (AZT), didanosine (ddI), lamivudine (3TC), stavudine (d4T), and zalcitabine (ddC). These nucleosides are metabolized to the triphosphate forms that competitively inhibit reverse transcriptase. Nonnucleoside reverse transcriptase inhibitors include nevirapine, delavirdine, and efavirenz. Combinations of agents are used because resistance, through mutations in the reverse transcriptase sequence, frequently arises during monotherapy.

• Question 713:

  A 20-year-old woman admits herself to the emergency room with a yellow discoloration of the whites of her eyes. She says that she does not drink and that she has not experienced any changes in her stool. Her liver enzyme profile and direct serum bilirubin levels are normal, while total bilirubin is elevated.

  What is the most likely cause for her jaundice?

  A. defect in hepatocytes
  B. defect in Kupffer cells
  C. gallstones
  D. hemolysis
  E. tumor obstructing bile duct
  D. hemolysis

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  Explanation

  Section: Physiology Rupture of large numbers of red blood cells can result in jaundice in the absence of any liver disease. The capacity of the liver to clear released heme metabolites such as bilirubin is temporarily exceeded. Since the liver will not perform its normal function to conjugate bilirubin before excretion in bile and urine, unconjugated bilirubin backs up in serum. This is the reason for the increase in total bilirubin, while conjugated bilirubin, also called direct bilirubin, is not affected. Direct bilirubin would be elevated with most defects of liver cells (choice A) and with obstruction of bile flow by gallstones (choice C) or by a tumor (choice E). All these would most likely also affect bile formation and consequently absorption of dietary fat and fat-soluble vitamins, with the consequence of developing oily, fatty stool. Kupffer cells (choice B) are macrophages within the liver and don't play a role in the production of bilirubin.

• Question 714:

  After removal of cancerous lymph nodes from the lateral pelvic wall, a patient develops painful spasms of the adductor muscles and sensory deficits in the medial thigh region. The adductor muscles are innervated by which of the following nerves?

  A. femoral
  B. inferior gluteal
  C. obturator
  D. pudendal
  E. sciatic
  C. obturator

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  Explanation

  Section: Anatomy The obturator nerve innervates the adductor muscles and the medial region of the thigh. The nerve originates from the lumbar plexus, runs on the lateral aspect of the pelvic wall, and exits through the obturator canal to reach the medial aspect of the thigh. Lying on the lateral pelvic wall, it may be injured by surgical mishap. The femoral nerve (choice A) innervates the anterior aspect of the thigh and the muscles contained within the sartorius and the quadriceps femoris. The inferior gluteal nerve (choice B) innervates the gluteus maximus muscle and is confined to the gluteal region. The pudendal nerve (choice D) is sensory to the genitalia, motor to the perineal muscles, the external urethral sphincter, and the external anal sphincter. The sciatic nerve (choice E) innervates the hamstring muscles in the posterior aspect of the thigh.

• Question 715:

  A 52-year-old woman tells her doctor that she is experiencing pains in the abdomen and thighs and insists that she is suffering from some form of cancer. A physical examination reveals only the presence of extensive varicose veins. While collecting additional patient history, you find that the woman has become increasingly anxious, with a sense of impending doom, and intensifying abdominal discomfort. You decide to do further medical workup to rule out cancer. Which of the following malignancies is most likely in this patient?

  A. carcinoma of the pancreas
  B. glioblastoma
  C. carcinoma of the stomach
  D. Kaposi sarcoma
  E. pancoast tumor
  A. carcinoma of the pancreas

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  Explanation

  Section: Behavioral Science and Biostatics Carcinoma of the tail of the pancreas often presents with symptoms of depression. Although other carcinomas and neoplasms (choices B, C, D, and E) may also cause depression, pancreatic cancer has to be ruled out first in a patient with severe depression.

• Question 716:

  A 42-year-old man presents with hepatomegaly, jaundice, refractory ascites, and renal insufficiency. Peripheral leukocytes exhibit only 20% of normal glucocerebrosidase activity. Which of the following would explain his symptoms?

  A. Fabry disease
  B. Gaucher disease
  C. Krabbe disease
  D. Niemann-Pick disease type II C
  E. Tay-Sachs disease
  B. Gaucher disease

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  Explanation

  Section: Biochemistry Numerous severe disorders are associated with the inability to properly degrade the complex carbohydrate moieties of glycosaminoglycans, proteoglycans, and glycoproteins. These disorders fall into a broad category of diseases termed the lysosomal storage diseases. Several of the lysosomal storage diseases result in hepatosplenomegaly, renal dysfunction, and skeletal defects, and therefore these symptoms are not diagnostic in themselves of a particular lysosomal storage disease, but only indicative of such disorders. However, disorders such as NPD (choice D) and Tay-Sachs disease (choice E) are of such severity that early childhood mortality occurs and thus would not present in a 42- year-old patient. It is necessary to evaluate enzyme function in skin fibroblasts or white cells of the blood. Gaucher disease is caused by a defect in glucocerebrosidase activity and hence an assayable decrease in the activity of this enzyme would be diagnostic of this disease. Fabry disease (choice A) results from a defect in alpha- galactosidase A. Krabbe disease (choice C) results from a defect in galactocerebrosidase. Based on genetic linkage analyses as well as enzyme studies and the characterization of accumulating lysosomal substances, NPD (choice D) should be divided into type I and type II; type I has two subtypes, A and B (NPA and NPB), which show deficiency of acid sphingomyelinase. NPD type II likewise has two subtypes, types C1 and C2 (NPC) and type D (NPD), both of which show accumulation of abnormal LDL-cholesterol. The NPC1 gene contains regions of homology to mediators of cholesterol homeostasis suggesting why LDL-cholesterol accumulates in lysosomes of afflicted individuals. Tay-Sachs disease (choice E) results from a defect in hexosaminidase A.

• Question 717:

  Which of the following drugs is useful in treatment of gout with recurrent renal urate stones because it decreases the excretion of uric acid?

  A. allopurinol
  B. aspirin
  C. colchicines
  D. indomethacin
  E. sulfinpyrazone
  A. allopurinol

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  Explanation

  Section: Pharmacology Allopurinol and its metabolite alloxanthine inhibit xanthine oxidase, thus preventing conversion of xanthine and hypoxanthine to uric acid. Although xanthine and hypoxanthine then accumulate, these compounds are more soluble than uric acid and less likely to deposit in joints or precipitate in the urine. Most doses of aspirin (choice B) increase retention of uric acid, especially low doses. Colchicine (choice C) is an inhibitor of microtubule function that brings relief in an acute gout attack by inhibiting the motility of granulocytes and preventing the formation of mediators of inflammation by leukocytes. Because of its toxicity at higher doses, it is now used chiefly at low doses to prevent acute attacks. Indomethacin (choice D) is an NSAID that inhibits COX and reduces formation of prostaglandins and eicosanoids involved in gouty arthritis. It has no effect on the formation of uric acid and very little on its excretion. Sulfinpyrazone (choice E) and probenecid are uricosuric agents--they increase the excretion of uric acid by the kidney. Renal uric acid excretion is determined by the balance between the amount filtered plus that actively secreted and the amount undergoing passive and active reabsorption. At very low doses, these agents inhibit active secretion and thus promote retention of uric acid. At higher (clinical) doses, both active secretion and active reabsorption are inhibited, with the result that excretion is enhanced.

• Question 718:

  A 47-year-old man has been chronically ill for the past 18 months. He undergoes a lung biopsy and a ection is shown in below figure. Which of the following is the most likely diagnosis?

  

  A. aspergillosis
  B. leprosy
  C. pneumocystosis
  D. sarcoidosis
  E. tuberculosis
  E. tuberculosis

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  Explanation

  Section: Pathology and Path physiology Figure shows a microscopic section of lung in which there are a number of necrotizing granulomas- containing giant cells. Aspergillosis (choice A) and pneumocystosis (choice C) are not associated with granuloma formation. Leprosy (choice B) does (in tuberculoid leprosy) form granulomas and may be found in the upper airways, but does not involve viscera such as the lung. Sarcoidosis (choice D) is a granulomatous disease of unknown etiology that can involve the lung and many other organs but the granulomas are non-necrotizing.

• Question 719:

  A 51-year-old alcoholic man is admitted to the emergency room with a 6-hour history of severe epigastric pain that radiates to his back and is more intense in a supine position. Physical examination reveals tachycardia, hypotension, and low-grade fever consistent with the early stage of shock. Which of the following serum measurements would be most useful in providing a diagnosis of his condition?

  A. amylase
  B. aspartate aminotransferase
  C. bilirubin
  D. calcium
  E. troponin I
  A. amylase

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  Explanation

  Section: Pathology and Path physiology The hallmark of acute pancreatitis is abdominal pain that is initially localized to the epigastrium and later becomes diffuse. It typically radiates to the back and is frequently more intense in the supine position. Other common symptoms and signs include nausea, vomiting, and tachycardia. If the pancreatitis is severe, hypotension can occur due to extravasation of blood and fluids into peritoneal (third) spaces and shock may ensue. Measurement of both amylase and lipase enzyme levels are useful in the diagnosis of acute pancreatitis. Amylase levels rise early in the course of the disease, while elevation of lipase levels may not occur until 24 hours after onset of illness. Although false positives and false negatives do occur, an elevated amylase level in conjunction with a consistent pattern of acute epigastric/abdominal pain is considered evidence of acute pancreatitis if other acute surgical conditions have been ruled out. Aspartate aminotransferase (choice B) is produced by a number of tissues and is increased in a variety of diseases; however, its greatest value is in liver function testing. Bilirubin (choice C) may be increased in an alcoholic but has no value in the diagnosis of acute pancreatitis. Calcium soap formation in the areas of fat necrosis may lead to a decrease in serum calcium (choice D) levels, but this is considered to be a nonspecific test. Troponin I (choice E) is now the preferred serum marker for the diagnosis of MI and has no role in the diagnosis of acute pancreatitis.

• Question 720:

  A person has an elevated plasma osmolality and reduced plasma ADH level and excretes a large volume of osmotically dilute urine. The urine contains no glucose. What is the most likely explanation for this situation?

  A. congestive heart failure
  B. nephrogenic diabetes insipidus
  C. neurogenic diabetes insipidus
  D. primary polydipsia
  E. uncontrolled diabetes mellitus
  C. neurogenic diabetes insipidus

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  Explanation

  Section: Physiology Normally, an elevated plasma osmolarity will stimulate increased ADH secretion and cause increased renal water reabsorption, which should lower the elevated plasma osmolarity. Since in this case ADH is reduced in the face of elevated osmolarity and the kidney is not reabsorbing water, it is clear that neurogenic diabetes insipidus is present (choice C). Choice A is incorrect because congestive heart failure will trigger water retention via the reduced cardiac output and lower arterial pressure, which stimulates ADH release via arterial baroreceptors. Choice B is incorrect because nephrogenic diabetes insipidus results from renal insensitivity to normal, or elevated circulating ADH levels. Choice D is incorrect because primary polydipsia is associated with reduced plasma osmolarity which will suppress ADH release. Choice E is incorrect because diabetes mellitus will be associated with elevated plasma and urine glucose levels.