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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 331:

  Creatinine clearance is often used to evaluate glomerular function. Which of the curves in below figure best represents the relationship between plasma creatinine concentration and creatinine clearance in a normal healthy person?

  

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

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  Explanation

  Section: Physiology Creatinine clearance is independent of plasma creatinine concentration; otherwise, creatinine would not be a useful measure of GFR. Clearance is defined as the amount of plasma that delivered the excreted substance, and for a substance that is neither actively secreted nor reabsorbed by the kidneys; its clearance equals the amount of plasma filtered through the glomerular membrane. All creatinine contained in that amount of plasma is excreted by the kidney, no matter what the concentration of creatinine in that plasma volume was. Because in the normal person a small amount of creatinine is secreted by the renal tubules, clearance at low plasma concentrations is slightly higher than at elevated plasma concentration (slight initial upward bend of curve). Curve Adescribes the relationship between creatinine plasma concentration and renal excretion of creatinine. Note that excretion and clearance are not synonymous. Curves B and D describe the clearance of a substance that is secreted and filtered, or filtered and reabsorbed, respectively. At large plasma concentration, the active transporters become saturated and the clearance of these substances approaches the creatinine clearance. Curve E depicts an improbable event with relatively increased clearance at both low and high concentrations of a substance, although there is an independent linear clearance at intermediate substance concentrations.

• Question 332:

  Which of the following will be observed in a person who is deficient in interleukin-1 synthesis?

  A. difficulty with immunoglobulin isotypic switching
  B. dependence on endotoxin for the induction of febrile responses
  C. effective mediation of acute phase responses
  D. effective regulation of vascular shock
  E. tendency to develop intense inflammatory reactions
  B. dependence on endotoxin for the induction of febrile responses

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  Explanation

  Section: Microbiology/Immunology Interleukin-1 (IL-1) is a protein generated primarily by macrophages. The cytokine is called endogenous pyrogen because it acts on the hypothalamus to produce fever during infections and inflammatory conditions. Thus individuals who are unable to produce IL-1 most likely will have to depend on endotoxin, the exogenous pyrogen, to induce febrile reactions. Mediation of immunoglobulin isotypic switching (choice A) requires IL-4 and IL-5. Acute-phase responses (choice C) and shock (choice D) are mediated by IL-6 and TNFalpha, TNF-alpha. Intense inflammatory reactions are produced by IL-1 (choice E).

• Question 333:

  A 32-year-old male cyclist is struck at night by a hit-and-run motorist. He is unconscious and severely injured and is not discovered until a pedestrian walks by 2 or 3 hours later. When he arrives at the emergency room, he is in shock and his BP is 80/30 mm Hg. He is transfused and a large wound on his right leg is cleaned and sutured. However, by the next day his urine output has decreased to less than 20 mL/h. Which of the following microscopic descriptions best describes his kidneys at this time?

  A. acute PMN infiltration of tubules and interstitium
  B. fibrinoid necrosis of arterioles
  C. focal tubular necrosis with pigmented cellular casts
  D. interstitial mononuclear infiltrate with "thyroidization"
  E. wedge-shaped areas of coagulative necrosis
  C. focal tubular necrosis with pigmented cellular casts

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  Explanation

  Section: Pathology and Path physiology One of the consequences of shock is hypoperfusion of the kidneys producing ischemia and acute tubular necrosis, which in turn produces acute renal failure. In this patient this was expressed in the greatly reduced urine production. The pigmented casts are formed from Tamm-Horsfall protein and the sloughed tubule cells. Acute PMN infiltration of tubules and interstitium (choice A) may be observed in acute pyelonephritis. Fibrinoid necrosis of arterioles (choice B) is seen in both malignant hypertension and in some vasculitides such as polyarteritis nodosa. Interstitial mononuclear infiltrate with "thyroidization" (choice D) would be an expected finding for chronic pyelonephritis. Wedge- shaped areas of coagulative necrosis (choice E) describe typical renal infarcts.

• Question 334:

  The hands and nails of a 45-year-old woman show the discolorations indicated in below figure. The woman states that she had pulmonary tuberculosis about 10 years ago. The clinician prescribes cortisol, but counsels her that as a side effect it might lead to thin skin and thinning of her limbs because of an increase in which of the following?

  

  A. adrenocorticotropic hormone (ACTH) secretion
  B. corticotrophin-releasing hormone (CRH) secretion
  C. insulin sensitivity in muscle
  D. protein degradation
  E. wound healing
  D. protein degradation

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  Explanation

  Section: Physiology Tuberculosis is known to increase the susceptibility of developing chronic adrenal insufficiency, also known as hypocortisolism, or Addison disease. It leads to hyperpigmentation as seen in the palmar skin creases and nails of the woman. Administration of cortisol replaces the inadequate release of glandular cortisol. Cortisol decreases protein synthesis and increases protein degradation. The newly available amino acids are then used for gluconeogenesis, which lead to the name "glucocorticoid" for cortisol.

  Increasing serum cortisol levels will decrease, not increase, secretion of ACTH (choice A) and CRH (choice B) by feedback inhibition. Cortisol decreases, not increases, the sensitivity of cells to insulin (choice C) by decreasing the translocation of glucose transporters to the cell membrane. Because cortisol decreases utilization of glucose by muscle and adipose tissue, blood glucose rises. This provides a stimulus to release insulin. Thus, prolonged exposure to high levels of cortisol can lead to diabetes mellitus due to exhaustion of pancreatic beta cells. Although cortisol acts as an anti- inflammatory agent, it has a negative, not a positive, effect on wound healing (choice E). Cortisol suppresses the activity of leukocytes which are important for debriding the wound. Cortisol increases catabolism, which leads to the breakdown of nutrients that are needed for generating new tissue. Cortisol stimulates catecholamines leading to vasoconstriction and consequent reduction of oxygen, nutrient, and white blood cell delivery. Cortisol may also inhibit collagen production.

• Question 335:

  Exhibit: *missing* In below figure, which point depicts the renal filtered glucose in an 11-year-old male with untreated Type I diabetes mellitus?

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

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  Explanation

  Section: Physiology Type I diabetes mellitus is a condition in which the pancreas secretes inadequate quantities of insulin, causing a fasting hyperglycemia. Choice Adepicts the filtered glucose in such a hyperglycemic state. Choices B and C depict the reabsorbed and excreted glucose which would also occur in a hyperglycemic state. Choice D depicts the filtered and reabsorbed glucose in a fasting, normoglycemic state. Choice E depicts the zero excreted glucose that would occur in a fasting nondiabetic individual since the tubular maximum for glucose has not been exceeded.

• Question 336:

  A 21-year-old woman presents with a 6-hour history of left-sided lower abdominal pain and is found to be hypotensive. A hemorrhagicmass is discovered in her left fallopian tube during laparoscopy. The tube is surgically excised. A photomicrograph of the tubal contents is displayed in below figure. Which of the following is the most likely diagnosis?

  

  A. chorioadenoma destruens
  B. choriocarcinoma
  C. ectopic tubal pregnancy
  D. granular cell tumor
  E. leiomyoma
  C. ectopic tubal pregnancy

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  Explanation

  Section: Pathology and Path physiology figure displays viable benign chorionic villi that are diagnostic of an ectopic tubal pregnancy. The clinical history of acute lower abdominal pain and hypotension are expected findings with an ectopic pregnancy. Chorioadenoma destruens (choice A) is also referred to as invasive mole, and would display invasion of chorionic elements into the uterine, not tubal, muscular layers. Choriocarcinoma (choice B) is a malignant neoplasm composed of gestational trophoblastic tissue. Figure displays benign chorionic elements. Granular cell tumor (choice D) is a neoplasm of neuraltype tissue. The photomicrograph is incompatible with this diagnostic consideration. Leiomyomas (choice E) rarely occur in the fallopian tube. They are most commonly seen in the wall of the uterus. Bundles of benign smooth muscle cells are seen microscopically.

• Question 337:

  Which of the following chemotherapeutic agents' mechanisms of action involve inhibition of thymidylate synthase and incorporation into RNA?

  A. anastrozole
  B. cytarabine
  C. doxorubicin
  D. 5-fluorouracil
  E. imatinib
  D. 5-fluorouracil

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  Explanation

  Section: Pharmacology The pyrimidine analog 5-fluorouracil (5-FU) is metabolized by ribosylation and phosphorylation to the nucleotide level (F-UMP). F-UMP is further metabolized to F-dUMP, an inhibitor of thymidylate synthase. Cells then become starved for TTP and incorporate FdUTP and dUTP in its place in DNA. 5- FU also becomes incorporated in RNA, leading to inhibition of RNAprocessing. Anastrozole (choice A) is a nonsteroidal inhibitor of aromatase, an enzyme required for synthesis of estrogens. This drug is useful against advanced estrogen or progesterone receptor-positive breast cancer. Cytarabine (cytosine arabinoside, ara-C, choice B), is an S-phase-specific antimetabolite that is metabolized to the triphosphate form, which blocks DNA synthesis. It is used exclusively in acute myelogenous leukemia. Doxorubicin (choice C) is an anthracycline antibiotic that intercalates into DNA causing strand breakage and blockage of both DNA and RNA synthesis. This agent is widely used in combination regimens for breast, endometrial, ovarian, testicular, and thyroid carcinomas, and several sarcomas and lymphomas. Imatinib (choice E) is an inhibitor of the tyrosine kinase activity of the Bcr-Abl oncogene product. It is a drug of choice in chronic myelogenous leukemia with the Philadelphia chromosome translocation.

• Question 338:

  According to Piaget, in which of the following do children of middle teenage years typically engage?

  A. abstract thinking
  B. circular reactions
  C. cognitive map
  D. concrete operations
  E. preoperational activities
  A. abstract thinking

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  Explanation

  Section: Behavioral Science and Biostatics According to Piaget, at about the time of puberty (which is on the average about age 11 in girls), the final stage of maturation in cognitive functioning, the period of formal operations, occurs. The individual's thought processes become more flexible, and transcend the here and now. Propositional thinking and hypothesis testing are prominent features of this stage.

• Question 339:

  A patient with untreated diabetes mellitus is admitted to the hospital for treatment. Laboratory findings include a blood pH below 7.2. Which of the following is likely to directly result from this level of acidemia?

  A. arteriolar constriction
  B. decreased catabolism
  C. hyperkalemia
  D. hypoventilation
  E. insulin sensitivity
  C. hyperkalemia

• Question 340:

  Achild born and raised in Chicago planned to spend the summer on a relative's fruit farm and help with the harvest. The summer passed uneventfully, but several days after the harvest began the child became jaundiced and very sick. On admission to the hospital the following clinical findings were recorded: in addition to the expected hyperbilirubinemia, the patient was hypoglycemic, had a markedly elevated rise in blood fructose concentration, and was hyperlactic acidemic. Further history taking revealed that during the harvest it was customary for the family to indulge in fruit-filled meals and to snack freely on fruit while carrying out the harvest. The following conclusions were reached. The elevated blood fructose was due to which of the following?

  A. an allergic reaction to constituents in the fruit diet
  B. defective hepatic fructokinase
  C. defective hepatic glucokinase
  D. defective hepatic fructose-1-phosphate aldolase (aldolase B)
  E. defective hepatic fructose-1,6-bisphosphate aldolase (aldolase A)
  D. defective hepatic fructose-1-phosphate aldolase (aldolase B)

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  Explanation

  Section: Biochemistry Patients with hereditary fructose intolerance have defective function in hepatic fructose-1-phosphate aldolase (also called aldolase B). This enzyme hydrolyzes fructose-1-phosphate to glyceraldehyde and dihydroxyacetone phosphate. The reaction is the second in the hepatic pathway of fructose metabolism, the initial one being the phosphorylation of fructose at the 1 position by fructokinase (choice B). When fructose is high in the diet (as in the consumption of large quantities of fruit) the capability to divert the fructose into the glycolytic pathway is severely impaired. Fructose becomes trapped in the liver as fructose- 1-phosphate. Due to the lack of aldolase B, the capacity to phosphorylate fructose becomes limiting due to feedback inhibition of fructokinase, resulting in an elevation in serum fructose levels. Hepatic glucokinase (choice C) and aldolase A (choice E) are not involved in the metabolism of fructose. An allergic reaction (choice A) would not manifest with elevated serum fructose, hyperbilirubinemia, or hyperlacticacidemia.