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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 591:

  Sean is 8 years old. He is referred by his school because he is habitually disruptive in class. Which of the following is the most useful area to explore at this point?

  A. his attention span
  B. his criminal record
  C. his relationship with his mother
  D. history of cruelty to animals
  E. history of enuresis
  A. his attention span

  ---

  Explanation

  Section: Behavioral Science and Biostatics Acommon cause of disruptiveness is attention deficit hyperactivity disorder (ADHD), which is characterized by inattention and hyperactivity. Enuresis (choice E), cruelty to animals (choice D), and trouble with the law (choice B) are often found in the childhoods of those who are eventually diagnosed as having an antisocial personality, but disruptiveness in class is more indicative of hyperactivity. Exploring Sean's relationship to his mother (choice C) may be useful, but not as much as assessing his attention span.

• Question 592:

  Which of the following statements most correctly reflects the effect of epinephrine stimulation of adipocytes?

  A. enzyme-dependent addition of fatty acids to glycerol-3-phosphate
  B. enzyme-dependent stepwise release of fatty acids from triglycerides
  C. increased synthesis and activation of LPL
  D. increased synthesis of HDL to transport fatty acids to peripheral tissues
  E. release of glycerol-3-phosphate for gluconeogenesis in the liver
  B. enzyme-dependent stepwise release of fatty acids from triglycerides

  ---

  Explanation

  Section: Biochemistry When epinephrine stimulates adipocytes (by interaction with beta-adrenergic receptors), the immediate response is the activation of adenylate cyclase, which in turn leads to an increase in cAMP. Increased levels of cAMP lead to activation of PKA, which in adipocytes will phosphorylate and activate hormonesensitive lipase. When hormone-sensitive lipase is active, it removes fatty acids in a stepwise manner from stored triacylglycerides. The released fatty acids then enter the blood and are bound by albumin for transport to peripheral tissues including the liver. LPL (choice C) is present on the surfaces of vascular endothelial cells and is activated by apo CII present in VLDLs and chylomicrons. Epinephrine is a stimulus that leads to energy consumption and therefore would not lead to an increase in triacylglyceride synthesis (choice A). When triacylglyceride breakdown is activated in adipocytes, the glycerol backbone is released as free glycerol, not glycerol-3-phosphate (choice E) to the blood and delivered to the liver for gluconeogenesis. Fatty acids released from adipose tissue are transported in the blood bound to albumin, not within any lipoprotein particle (choice D).

• Question 593:

  A 2-day-old male infant has not passed any meconium and is now developing signs of obstruction. Examination of the colon would reveal which of the following abnormalities?

  A. absence of parasympathetic ganglion cells in the submucosal and myenteric plexus
  B. absence of the nerve fibers that innervate the wall
  C. atrophy of the mucosal lining of the wall
  D. hypertrophy of the muscle coat of the wall
  E. presence of multiple small polyps along the mucosal surface
  A. absence of parasympathetic ganglion cells in the submucosal and myenteric plexus

  ---

  Explanation

  Section: Pathology and Path physiology Hirschsprung disease is caused by the congenital absence of parasympathetic ganglion cells in the submucosal and myenteric plexus. This presents clinically soon after birth as an inability to pass stool and abdominal distention. The diagnosis is usually confirmed by a full-thickness colon biopsy showing disorganized, nonmyelinated nerve fibers replacing the missing ganglion cells. In Hirschsprung disease, the ganglion cells, not the nerve fibers (choice B), are missing. Muscular hypertrophy (choice D) and atrophy (choice C) are not specific diagnostic findings with Hirschsprung disease. Mucosal polyp development (choice E) is not associated with Hirschsprung disease.

• Question 594:

  Which of the following agents used to prevent organ transplant rejection is most likely to cause significant decrease in creatinine clearance?

  A. antilymphocyte globulin
  B. cyclosporine
  C. mycophenolate mofetil
  D. prednisone
  E. sirolimus
  B. cyclosporine

  ---

  Explanation

  Section: Pharmacology Cyclosporine and tacrolimus (not listed) are associated with nephrotoxicity as a major adverse effect. Antilymphocyte globulin (choice A) is associated with foreign protein reactions, including anaphylaxis. Mycophenolate mofetil (choice C) causes GI disturbances and myelosuppression. Prednisone (choice D) causes a variety of chronic toxicities conveniently summarized as "Cushingoid effects" but has little acute toxicity. Sirolimus (choice E) is associated with bone marrow suppression.

• Question 595:

  Alaboratory test for a newly discovered infectious disease is found to have a sensitivity of 90% and a specificity of 90%. The disease has a prevalence of 1%. The test is run on one of your patients and yields a negative result. What is the probability (to the nearest whole number) of this result being a true negative rather than a false-negative result (what is the predictive value negative)?

  A. 1%
  B. 10%
  C. 50%
  D. 90%
  E. 100%
  E. 100%

  ---

  Explanation

  Section: Pathology and Path physiology In a population of 100,000, 1000 people will have the disease and 99,000 will be disease free (the prevalence of the disease equals 1%). The sensitivity equals 90%, which indicates that in a population of people with the disease, 90% will have a positive test result (true positives) and 10% will have a negative test result (false negatives). The specificity also equals 90%, which indicates that in a population of people without the disease, 90% will have a negative test result (true negatives) and 10% will have a positive test result (false positives). Thus, looking at the negative test results, there were 100 false negatives (10% of the 1000 people with the disease) and 89,100 true negatives (90% of the 99,000 people without the disease). Therefore the chances of the test result being a true negative = (true negatives)/(true negatives + false negatives) = (89,100)/(89,100 + 100) = 99.9% (= 100% to the nearest whole number). Mathematically, 1% (choice A), 10% (choice B), 50% (choice C), and 90% (choice D) can be eliminated.

• Question 596:

  A 26-year-old man presents with a 3-week history of increasing pain just below his right knee. He does not recall sustaining any trauma to his leg and is not experiencing pain elsewhere; he states that he is otherwise healthy. Examination reveals only tenderness to palpation in the area. An x-ray of his right knee demonstrates a small lytic lesion in the tibial medial condyle surrounded by a focus of sclerosis. Based on this information, what is the most likely diagnosis?

  A. gout
  B. osteochondroma
  C. osteomyelitis
  D. osteosarcoma
  E. rheumatoid arthritis
  C. osteomyelitis

  ---

  Explanation

  Section: Pathology and Path physiology Bacteria are the principle cause of osteomyelitis, infecting bone either by direct penetration (e.g., puncture), extension from a surrounding area (e.g., cellulitis), or via hematogenous spread of an overt infection (e.g., sinusitis, pneumonia) or an occult bacteremia (e.g., mouth, intestinal flora); the leading causative pathogen is S. aureus. Long bone metaphyses are commonly infected in children, whereas long bone epiphyses and vertebrae are more typically affected in adults. Presentation may be limited to pain, with or without fever. The classic x-ray findings include lytic changes caused by necrotic bone (sequestrum) and sclerosis due to the ingrowth of fibrous tissue as new bone growth (involucrum) forms around the devitalized bone. Gout (choice A) is associated with hyperuricemia, leading to acute synovial inflammation and deposition of urate crystals in joint spaces; similarly, rheumatoid arthritis (choice E) is an autoimmune disorder affecting the synovial lining of various joints (chronic synovitis).

  Osteosarcomas (choice D) are typically large, bulky malignancies most commonly originating in long bone metaphyses, often with secondary spread into epiphyseal regions. Osteochondromas (choice B) are benign tumors arising near the growth plate in endochrondrally derived long bones which form bony, pedunculated, cartilage-capped lesions.

• Question 597:

  A 7-year-old child has been exhibiting impulsivity and disruptive behaviors at school. When further evaluation reveals that the child has been unable to sleep at night for at least 3 months out of fear that his parents may divorce, which of the following is an important diagnostic possibility?

  A. anaclitic depression
  B. attention deficit hyperactivity disorder
  C. conduct disorder
  D. generalized anxiety disorder
  E. separation anxiety disorder
  E. separation anxiety disorder

  ---

  Explanation

  Section: Behavioral Science and Biostatics Separation anxiety disorder involves excessive anxiety concerning separation from the home or someone to whom the child is attached. The disturbance must continue for more than 4 weeks, with onset before age 18. Choices A, B, C, and D are not particularly indicated by the new information provided.

• Question 598:

  A female patient is diagnosed by sonogram with uterus didelphys (double uterus). The imaging study reveals a bicornuate uterus with a single vagina. During development, the uterus develops from which of the following?

  A. mesonephric ducts
  B. nephrogenic ridge
  C. paramesonephric ducts
  D. urogenital sinus
  E. yolk sac
  C. paramesonephric ducts

  ---

  Explanation

  Section: Anatomy During development, the uterus develops from fusion of the caudal ends of the para-mesonephric ducts. Failure of fusion results in uterus didelphys. The mesonephric ducts (choice A) give rise to the epididymis, vas deferens, and ejaculatory duct in the male. The nephrogenic ridge (choice B) is the derivative of the urogenital ridge which will give rise to the urinary system. The urogenital sinus (choice D) gives rise to the urinary bladder, urethra, vagina, and associated glands in the female. The yolk sac (choice E) plays a role in transfer of nutrients before uteroplacental circulation is established, in blood development before the liver begins its hemopoietic activity, and is the site of formation of primordial germ cells.

• Question 599:

  Which of the following agents used in treating testicular cancer, requires hydration and dieresis to prevent renal toxicity?

  A. bleomycin
  B. cisplatin
  C. cyclophosphamide
  D. 5-fluorouracil
  E. paclitaxel
  B. cisplatin

  ---

  Explanation

  Section: Pharmacology Cisplatin binds to DNA where it forms intra- and interstrand crosslinks. Cisplatin is particularly effective in testicular and ovarian cancers in combination with other antitumor agents. Cisplatin exerts a renal toxicity that may be prevented by the infusion of saline to maintain a high urine flow. Ototoxicity involving high- frequency hearing loss is an effect that is not prevented by hydration. The natural product bleomycin (choice A) binds to DNA and causes single- and double-strand breaks, leading to cytotoxicity.

  The drug is particularly useful against Hodgkin lymphoma and testicular tumors. Bleomycin has the serious toxicity of pulmonary fibrosis. Cyclophosphamide (choice C) is widely used in combination regimens. Nausea and vomiting are the most common toxicities. Hemorrhagic cystitis may be minimized by hydration and use of the drug mesna. Note that this toxicity is not at the level of the kidney. The pyrimidine analog 5-fluorouracil (5-FU) (choice D) is used to treat a wide variety of carcinomas. Toxicity from 5-FU is expressed as GI disturbances (anorexia, nausea, stomatitis, and diarrhea) and myelosuppression. Paclitaxel (choice E) is particularly useful in treating metastatic breast and ovarian cancer. The primary toxicity of paclitaxel is bone marrow suppression.

• Question 600:

  The activity of which structure in the pons is suppressed by opioids, alpha-2-agonists, and gammaaminobutyric acid (GABA), and produces most of the noradrenergic input to the brain?

  A. anterior Cingulate Gyrus
  B. locus ceruleus
  C. mammillary bodies
  D. nucleus pulposus
  E. nucleus solitarius
  B. locus ceruleus

  ---

  Explanation

  Section: Behavioral Science and Biostatics The locus ceruleus, located in the pons, produces most of the noradrenergic input to the brain, and has receptors for opioids and autoreceptors for norepinephrine (?2) as well as GABA. The locus ceruleus seems to be involved in alertness and anxiety response. Choices A, C, D, and E refer to structures elsewhere.