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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 641:

  Referring to following figure, arrow 2 indicates which of the following structures?

  

  A. intercalated disk
  B. motor end-plate
  C. sarcoplasmic reticulum
  D. tendinous junction
  E. transverse tubule or T tubule
  A. intercalated disk

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  Explanation

  Section: Anatomy The intercalated disks are specialized junctional complexes found only in cardiac muscle and they appear as dark lines between the muscle fibers. The motor end-plate (choice B) is a specialized group of synapses between the axon terminals of a motor neuron and the sarcolemma of a skeletal muscle fiber. It is not seen in cardiac muscle. The sarcoplasmic reticulum (choice C) is a specialized modification of the smooth endoplasmic reticulum for sequestering calcium ions. The transverse tubule or T tubule (choice E) is an invagination of the sarcolemma, which penetrates the muscle fiber and overlies the surface of the myofibrils. The sarcoplasmic reticulum and T tubule can only be seen in electron micrographs. There is no tendinous junction (choice D) in cardiac muscle.

• Question 642:

  A 24-year-old student is brought to the emergency room after being found in a ditch where he had lain overnight after being hit by a car. He complains of severe pain in the left arm and examination reveals a broken humerus. Neurological examination reveals that the patient can extend the elbow but displays inability to supinate the elbow when it is extended. The patient also has wrist drop and very weak hand grasp. The neurological lesion is likely localized at which of the following locations?

  A. posterior cord of the brachial plexus
  B. posterior divisions of the brachial plexus
  C. radial nerve at the distal third of the humerus
  D. radial nerve at the midforearm
  E. radial nerve at the wrist
  C. radial nerve at the distal third of the humerus

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  Explanation

  Section: Anatomy Wrist drop and weak hand grasp indicate that the patient has a lesion of the radial nerve, most likely at the distal third of the humerus, sparing the innervation of the tr<pdf2txt-u>韈</pdf2txt-u>eps brachii but affecting the supinator. Because the patient can extend the elbow, the integrity of the posterior cord (choice A) and posterior divisions (choice B) of the brachial plexus is preserved. Lesion of the radial nerve at the midforearm (choice D) and wrist (choice E) would spare the innervation of the supinator.

• Question 643:

  Altered plasma renin levels can occur in both normal and pathologic conditions. Which of the following states is associated with a decrease in plasma renin levels?

  A. heart failure
  B. primary aldosteronism
  C. renal artery stenosis
  D. salt restriction
  E. upright posture
  B. primary aldosteronism

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  Explanation

  Section: Physiology Most patients with primary aldosteronism (Conn syndrome) have an adrenal adenoma. The increased plasma aldosterone concentration leads to increased renal Na+ reabsorption, which results in plasma volume expansion. The increase in plasma volume suppresses renin release from the juxtaglomerular apparatus and these patients usually have low plasma renin levels. Secondary aldosteronism is due to elevated renin levels and may be caused by heart failure (choice A) or renal artery stenosis (choice C). Salt restriction (choice D) and upright posture (choice E) decrease renal perfusion pressure and therefore increase renin release from the juxtaglomerular apparatus.

• Question 644:

  Which of the following antiviral agents uses viral thymidine kinase as part of its activation pathway?

  A. acyclovir
  B. amantadine
  C. foscarnet
  D. saquinavir
  E. zidovudine
  A. acyclovir

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  Explanation

  Section: Pharmacology Acyclovir is an acyclic guanine nucleoside analog that undergoes activation by phosphorylation by herpesviral thymidine kinase (but not the host thymidine kinase) to form the acyclovir triphosphate form and causes DNA chain termination. Amantadine (choice B) is a tricyclic amine that inhibits uncoating of influenza A virus. It is also used for the treatment of parkinsonism. Foscarnet (choice C) is a pyrophosphate analog that inhibits herpesvirus nucleic acid synthesis at the level of the viral DNA polymerase.

  Saquinavir (choice D) is an HIV protease inhibitor. Zidovudine (choice E) is an antiretroviral thymidine analog. The triphosphate is a competitive inhibitor of reverse transcriptase and a DNA chain terminator. The monophosphate is a competitive inhibitor of thymidine kinase.

• Question 645:

  A 38-year-old woman has experienced the gradual onset of a goiter. Serum T4 and T3 are within the reference range and thyroid-stimulating hormone (TSH) is slightly increased. Serum antithyroid peroxidase (antimicrosomal) antibodies are detected but there are no TSH receptor antibodies. Which of the following is the most likely diagnosis?

  A. chronic autoimmune thyroiditis
  B. Graves disease
  C. primary atrophy of the thyroid
  D. riedel struma
  E. subacute thyroiditis
  A. chronic autoimmune thyroiditis

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  Explanation

  Section: Pathology and Path physiology Patients with chronic autoimmune thyroiditis or Hashimoto disease may demonstrate several different serum thyroid autoantibodies, but these vary from patient to patient. Thyroid peroxidase antibodies are found in roughly 85% of patients with Hashimoto disease, about 40% of patients with Graves disease, and in <15% of patients with other thyroid disorders. The absence of TSH receptor antibodies rules out Graves disease (choice B), making Hashimoto disease the most likely diagnosis. Primary atrophy of the thyroid (choice C) and Riedel struma (choice D) are not usually associated with the formation of thyroid autoantibodies. Thyroid autoantibodies are only found transiently in a few patients with subacute thyroiditis (choice E) making this choice unlikely.

• Question 646:

  Exhibit:

  

  Please refer to the exhibit.

  Which of the following proteins uses the sodium electrochemical gradient to actively transport a solute into the cell?

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

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  Explanation

  Section: Physiology SGLT-1 is a sodium-dependent cotransport protein that uses the sodium electrochemical gradient to actively move glucose into the cell. GLUT2 (choice B) does transport glucose, but it is a facilitated diffusion carrier that moves glucose down its concentration gradient. The Na, K-ATPase (choice C) is a primary active transport protein. It uses the energy liberated from ATP hydrolysis to actively transport sodium and potassium. It establishes and maintains the sodium electrochemical gradient. The sodium:

  calcium countertransport protein (choice D, also termed NCX) uses the sodium electrochemical gradient to move calcium ions out of, not into, the cell. The calcium ATPase (choice A, also termed PMCA) is a primary active transport protein that actively transports calcium out of the cell. Both NCX and PMCA serve to maintain a low intracellular calcium concentration.

• Question 647:

  A 28-year-old, 166 cm (5.45 ft) tall woman, weighing 170 kg (375 lbs) successfully underwent biliopancreatic diversion surgery, in which a portion of her stomach was removed and the remaining portion of the stomach was connected to the lower portion of the small intestine (see below figure). What is her prevalence for peptic ulcer disease and for what reason?

  

  A. it is higher due to inflammation caused by the surgical staples
  B. it is higher due to the loss of secretin release
  C. it is lower since chief cells were surgically resected
  D. it is lower since G cells were surgically resected
  E. it is unchanged since the surgery was successful
  D. it is lower since G cells were surgically resected

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  Explanation

  Section: Physiology Peptic ulcers refer to areas of the stomach or duodenal lining which became eroded by stomach acid. Stomach acid is produced by parietal cells which are stimulated by gastrin. Since gastrin-secreting G cells are primarily found in the gastric glands of the distal stomach (gastric antrum), the surgically removed stomach portion, the patient's prevalence for peptic ulcer disease is lower after the surgery, and not unchanged as stated in choice E. However, the surgery is a dramatic event and increases the risk

  for many GI problems, including symptoms resulting from nutritional deficiencies. Hence, the surgery is only used for morbidly obese people (the patient's body mass index is above 50), who had no success with diet and medication. Inflammation potentially triggered by the surgical staples (choice A) might trigger prostaglandin release which has a protective function of the stomach lining. Although secretin (choice B) is known to suppress gastric acid release by inhibiting gastrin release, the lower number of G cells after surgery does not make this a good choice. Chief cells (choice C) are primarily present in the proximal stomach, the portion that remains after surgery.

• Question 648:

  Exhibit:

  

  Please refer to the exhibit.

  Which of the following is the agent of choice for chronic treatment of simple hypothyroidism (myxedema)?

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

• Question 649:

  A 72-year-oldman with a known history of chronic essential hypertension dies unattended at home. The medical examiner determines the cause of death to be hypertensive intracerebral hemorrhage. The most likely site of the hemorrhage is which of the following?

  A. basal ganglia or thalamus
  B. cerebellum
  C. frontal lobe
  D. occipital lobe
  E. pons
  A. basal ganglia or thalamus

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  Explanation

  Section: Pathology and Path physiology About 65% of hypertensive intracerebral hemorrhages occur within the basal ganglia or thalamus. Chronic hypertension predisposes to weakening of arteriole walls with subsequent production of CharcotBouchard aneurysms. A rupture of these aneurysms frequently results in fatal intracerebral hemorrhage. The cerebellum (choice B) is the site of hypertensive hemorrhage about 8% of the time. Hypertensive hemorrhages are very rare in the frontal lobe (choice C) and in the occipital lobe (choice D). The pons (choice E) is the site of hypertensive hemorrhage about 15% of the time.

• Question 650:

  A 35-year-old man has severe schizophrenia. He is institutionalized and maintained on antipsychotic medication. Which of the following drugs is most likely to cause parkinsonian adverse effects?

  A. clozapine
  B. fluphenazine
  C. haloperidol
  D. olanzapine
  E. quetiapine
  C. haloperidol

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  Explanation

  Section: Pharmacology Haloperidol is a very potent and efficacious antipsychotic but also manifests the highest incidence of extrapyramidal effects. Clozapine (choice A), olanzapine (choice D), and quetiapine (choice E) are newer, atypical antipsychotic agents with much lower incidence of extrapyramidal toxicity. Fluphenazine (choice B) is a potent phenothiazine with an intermediate tendency to produce parkinson- like effects.