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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 751:

  A 73-year-old woman complains of difficulty sleeping, exercise fatigue, and shortness of breath. Examination reveals mental confusion, swollen ankles, pulmonary rales, and dyspnea while supine. Immediate treatment of the patient may be needed if signs and symptoms, especially those of pulmonary congestion, worsen rapidly. Which of the following would be most beneficial in treating acute, severe pulmonary edema?

  A. amiloride
  B. furosemide
  C. hydrochlorothiazide
  D. losartan
  E. metoprolol
  B. furosemide

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  Explanation

  Section: Pharmacology Rapid worsening of pulmonary congestion is suggestive of life-threatening acute cardiac decompensation (acute heart failure). Furosemide, a loop diuretic, is one of the most effective agents available because it has powerful diuretic action and also reduces pulmonary vascular pressures. Amiloride (choice A) is a much less efficacious diuretic with primary action in the collecting tubule of the nephron and little or no effect on pulmonary vessels. Hydrochlorothiazide (choice C) is a diuretic of intermediate efficacy that acts in the distal convoluted tubule. Losartan (choice D) is an angiotensin receptor blocker and is not effective in acute reduction of congestive symptoms. Metoprolol (choice E) is a betaadrenoreceptor blocker that is valuable in the long-term, chronic therapy of HF, but not effective(and usually contraindicated) in acute decompensation.

• Question 752:

  One of your diabetic patients has a blood glucose level of 200 mg/dL. Surprisingly, a dipstick test is negative for urinary glucose. How could this finding be explained?

  A. dipstick tests are more sensitive for reducing sugars other than glucose
  B. the patient has defective tubular glucose transporters
  C. the patient has diabetes insipidus
  D. the patient has significantly reduced GFR
  E. the patient is in a state of antidiuresis
  D. the patient has significantly reduced GFR

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  Explanation

  Section: Physiology Glucose excretion by the kidneys depends on the glomerular filtration and tubular reabsorption rates. Glucose first appears in the urine, when the capacity of the glucose transporters in the proximal tubuli cells is exceeded. This usually occurs at plasma glucose levels higher than 180 mg/dL. Patients with longstanding diabetes mellitus often have decreased renal function and reduced GFR. Under these circumstances the threshold (i.e., plasma level) for excretion of glucose is higher than in a healthy person.

  Urine dipsticks nowadays are both sensitive and specific for glucose, detecting as little as 100 mg/dL (choice A). Earlier dipstick tests were sensitive but not specific (they detected other reducing sugars in addition to glucose). Adefect in glucose transporters (choice B) results in glucosuria even at normal plasma glucose concentration. Patients with diabetes insipidus (choice C) have a large urine output due to absence of ADH or defective renal ADH receptors, but should not have a plasma glucose level of 200 mg/dL. Antidiuresis (choice E) increases the concentration of solutes in the urine and increases the sensitivity to detect urine glucose. Reabsorption of filtered glucose occurs in the proximal tubule by active transport.

• Question 753:

  Exhibit:

  

  Please refer to the exhibit:

  Which of the following is a feature of the principal cell in the cortical collecting duct?

  A. A
  B. B
  C. C
  D. D
  E. E
  D. D

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  Explanation

  Section: Physiology The principal cell is the major sodiumreabsorbing and potassium-secreting cell in the collecting duct. It functions via specific sodium and potassium channels that increase luminal cell membrane permeabilty to these ions and a sodium/potassium ATPase in the basolateral cell membrane. Choice A is incorrect because the specific movement of sodium through the luminal membrane via specific sodium channels creates an excess of negative charges in the tubular lumen. Choice B is incorrect because sodium is reabsorbed while potassium is predominantly secreted by this cell. Choice C is incorrect because aldosterone regulates sodium and potassium transport, not water transport. ADHdoes increase water transport via aquaporins. Choice E is incorrect because a sodium/potassium/2 chloride transporter is not located in this cell type, but is located in the thick ascending loop of Henle cells instead.

• Question 754:

  A 73-year-old man is brought to his primary care physician by his wife who is concerned about her husband's progressive memory problems over the past year. She notes that he originally began forgetting to do his normal household chores, and then started routinely misplacing things around the house; he has also forgotten to close or lock doors or turn off the oven and has left the car running in the garage. He is having trouble remembering the driving routes to familiar places and names of longtime friends and acquaintances. The patient's neurological examination reveals disorientation to place and time, inability to recall three objects, and difficulty following a threestage command. Based on this information, which of the following best explains the pathogenesis of his condition?

  A. cortical atrophy with neuronal cytoskeletal abnormalities and abnormal filamentous protein aggregation
  B. gray matter damage to mammillary bodies and hemorrhage associated with nutritional deficiency
  C. infiltrative mass of pleomorphic cells with areas of pseudopalisading, necrosis, and endothelial cell proliferation
  D. injury of oligodendroglial cells with focal areas of demyelination, mononuclear infiltrate, and patchy gliosis
  E. selective neuronal loss secondary to excitotoxicity associated with excessive glutamate levels
  A. cortical atrophy with neuronal cytoskeletal abnormalities and abnormal filamentous protein aggregation

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  Explanation

  Section: Pathology and Path physiology This patient's symptoms are very consistent with a diagnosis of AD, the most common cause of dementia in patients over 55. Over the course of this disease, patients suffer progressive impairment in higher cognitive functioning, including memory loss, spatial disorientation, language deficits, and mood and behavior changes. In end-stage AD, patients become rigid, mute, and bedridden. Definitive diagnosis of AD requires histological examination of the brain; however, with proficient clinical evaluation postmortem findings agree with clinical diagnosis in approximately 90% of cases. Grossly, brains affected by AD demonstrate cortical atrophy with widening of the sulci; changes are usually most pronounced in the frontal, temporal, and parietal lobes. With advanced disease, parenchymal atrophy results in compensatory dilation of the ventricles (hydrocephalus ex vacuo). The most severe pathology is found in the hippocampus and the nucleus basalis of Meynert of the anterior forebrain, which represent the main source of cholinergic input to the cerebral cortex. The characteristic microscopic lesions in AD are neuritic plaques and NFTs. Plaques consist of focal, spherical deposits of aggregated amyloid beta (A) fibrils surrounded by degenerating neuronal processes (dystrophic neuritis), reactive astrocytes and microglia; A plays a major role in the pathogenesis of AD. NFTs represent twisted bundles of filaments within neuronal cell bodies that extend into dendrites. Their chief constituent is a hyperphosphorylated tau protein, an axonal microtubuleassociated protein important for proper assembly and stability of microtubules, which are involved in the transport of essential molecules through the neuron. NFTs are also seen in neurodegenerative diseases other than AD. They are well demonstrated with silver stain and particularly affect pyramidal neurons (e.g., frontal cortex, hippocampus, amygdala) where they often have an elongated, flame-shaped appearance. Choice B refers to injury caused by thiamine deficiency in alcoholism; choice C describes the morphological appearance of a glioblastoma; choice D reflects changes seen in multiple sclerosis, and choice E describes biochemical changes that occur in cerebral ischemia.

• Question 755:

  End-stage chronic renal disease is associated with a glomerular filtration rate (GFR) of less than 15 mL/ min and an increase in which of the following blood values?

  A. calcium
  B. erythropoietin (EPO)
  C. hematocrit
  D. phosphate
  E. 1,25-(OH vitamin
  D. phosphate

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  Explanation

  Section: Physiology Chronic renal failure reduces phosphate excretion and thus phosphate accumulates in the blood. The renal peritubular cells are the major source of circulating EPO, which stimulates red blood cell production. Hence renal failure will decrease EPO levels (choice C) and will decrease hematocrit (choice E) as well. The kidney is responsible for production of vitamin D3, the active

  

  form of vitamin D, and renal failure also results in decreased active vitamin D (choice E). Secondary to this, the circulating levels of calcium will decrease (choice A).

• Question 756:

  HIV isolated from the same patient over time may differ antigenically. HIV also differs from the viruses found in the general class of RNA tumor viruses. Which of the following best explains how HIV differs from most other RNA tumor viruses?

  A. HIV contains the gag gene
  B. HIV contains the pol gene
  C. HIV contains two copies of singlestranded RNA in its virion
  D. HIV does not require T4 receptor protein for adsorption to host cells
  E. HIV lyses the host cells
  E. HIV lyses the host cells

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  Explanation

  Section: Microbiology/Immunology An important difference between the AIDS (HIV) virus and the RNA tumor viruses is that HIV lyses the host cells, while RNA tumor viruses transform the cells they invade, butthey lack cytolytic activity. The tropism of the HIV for T4 lymphocytes depends on the presence of the T4 protein on the surface of the lymphocytes. This protein serves as the receptor for the adsorption of the HIV to T4 lymphocytes. The HIV is a member of the retroviruses (choice D). The genomic RNA molecule of HIV contains the gag, pol, and env genes. Thus, the HIV does not differ from RNA tumor retroviruses (choices A and B). HIV contains one copy, not two, of single-stranded RNA in its virion (choice C).

• Question 757:

  Which theory concerning pain postulates that non-pain sensations, such as vibration or pressure, may affect the transmission and perception of pain sensation?

  A. bradykinin theory
  B. gate control theory
  C. pattern theory
  D. peptide theory
  E. specificity theory
  B. gate control theory

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  Explanation

  Section: Behavioral Science and Biostatics The theories involving pain perception are: (1) the specificity theory (choice E), postulating that there are specific pain receptors transmitting specific pain signals through specific neurons to specific areas of the brain; (2) the pattern theory (choice C), postulating the existence of "reverberating circuits" to explain phantom pain; and (3) the gate control theory (choice B), postulating an interaction between pain sensation and other sensations competing for transmission at the spinal cord level. Choices A and D do not postulate the interaction with non-pain sensations.

• Question 758:

  Below figure shows the amounts of water ingested and secreted daily into the GI tract by a healthy individual. Since about 100 mL of water is excreted daily in the stool, which of the following volumes (in milliliter) best reflects the daily absorbed water in the indicated areas x, y, and z of the GI tract?

  

  

  A. Option A
  B. Option B
  C. Option C
  D. Option D
  E. Option E
  B. Option B

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  Explanation

  Section: Physiology The small intestine absorbs massive quantities of water. Anormal person takes in about 2 L of fluid every day. Another 7 L of fluid are received by the small intestine daily as secretions from salivary glands (1.5 L), stomach (2 L), pancreas (1.5 L), liver (0.5 L), and the small intestine itself (1.5 L). That results in a total of about 9 L of fluid entering the GI tract daily. In a healthy person the duodenum, jejunum, and ileum absorb the bulk of the water that enters the GI tract, here 8500 mL. The colon absorbs most of the remaining fluid (400 mL), not the bulk of it (choice A) and excretes about 100 mL daily. There is generally no water absorption in the esophagus and stomach. Therefore, choices C, D, and E are incorrect.

• Question 759:

  A 66-year-old woman is undergoing surgery to repair an abdominal aortic aneurism. During the procedure the abdominal aorta is momentarily clamped below the renal arteries. below figure shows the left ventricular pressure/ volume relationship immediately before (curve 1) and during (curve 2) the aortic occlusion. The change in curve 2 represents an increase in which of the following?

  

  A. afterload
  B. cardiac output
  C. ejection fraction
  D. preload
  E. stroke volume
  A. afterload

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  Explanation

  Section: Physiology Clamping of the abdominal aorta suddenly increases peripheral vascular resistance to blood flow. Hence aortic pressure suddenly rises to a high level. This represents an increased afterload, that is, an increased ventricular pressure is required to eject blood into the aorta. below figure shows that momentary clamping of the aorta does not alter the preload, that is, the filling of the ventricle during diastole (choice D). below figure shows that the inverse relationship between afterload and stroke volume causes a reduced stroke volume (choice E) immediately after aortic clamping. Since end- diastolic volume (preload) is unchanged, while stroke volume is decreased, this represents a decreased ejection fraction (choice C). Cardiac output is calculated as stroke volume X heart rate. Since heart rate is not given, the cardiac output cannot be determined (choice B). However, it is likely, by Ohm's law that a momentary rise in peripheral vascular resistance will decrease cardiac output.

• Question 760:

  A 16-year-old boy fractured his leg while running track at school. X-ray studies revealed an abnormality at the lesion site indicating that this was a pathological fracture. Abiopsy of the area was taken and a photomicrograph of the tissue is shown in below figure. The most likely diagnosis is which of the following?

  

  A. chondroblastoma
  B. chondrosarcoma
  C. osteoid osteoma
  D. osteoblastoma
  E. osteochondroma
  F. osteosarcoma
  F. osteosarcoma

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  Explanation

  Section: Pathology and Path physiology This section in figure shows many large, hyperchromatic, pleomorphic cells that are producing osteoid, typical of an osteosarcoma. A chondroblastoma (choice A) would display sheets of primitive chondroblasts within a cartilage matrix that is irregularly calcified. Chondrosarcoma (choice B) most typically occur in the age range of 4060 with the most common locations being the pelvic girdle, ribs, shoulder girdle, and to a lesser extent the long bones, vertebrae, and sternum. Microscopically, the lower grade tumors will show chondroid differentiation whereas a grade 3 tumor may appear as a spindle cell tumor with little chondroid differentiation. Osteoid osteoma (choice C) and osteoblastoma (choice D) have a very similar microscopic appearance consisting of a random pattern of woven bone with many osteoblasts in evidence within a stroma of granulation-like tissue. Osteoid osteomas are small, very painful tumors that are found in teenagers and young adults and respond very well to aspirin. It usually occurs in the long bones of the leg. Osteoblastomas are larger tumors that more usually affect the vertebrae. Osteochondromas (choice E) (or exostoses) are mushroom-like growths that usually bud from the metaphysic of long bones but may occur, less frequently, at many other sites. Microscopically, one sees irregular trabecular bone covered by a cartilaginous cap.