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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 841:

  Which pair of organisms below can be routinely responsible for food poisoning?

  B. cereus
  C. tetani and
  C. tetani and

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  Explanation

  Section: Microbiology/Immunology

  C. perfringens (choice C) may well cause myonecrosis and gas gangrene in an invasive infection due to the many tissue-destroying toxins the bacteria can make. However, a powerful enterotoxin that induces intense diarrhea in 618 hours can be made by

  C. perfringens. It is similar to food poisoning caused by

  B. cereus (a soil organism) and tends to be self-limited. B cereus (choice C) produces an emetic type of food poisoning and a diarrheal form by toxins produced by the bacteria. It has a short incubation

  (hours) and recovery usually occurs in 24 hours.

  C. botulinum (choice A) produces a toxin that is ingested and affects the CNS in 24 days. Gastrointestinal anthrax (choice A) is quite rare.

  C. difficile (choice B) become predominant in the intestinal tract as a result of antibiotic treatment. It produces two toxins that cause pseudomembranous colitis.

  C. tetani (choice D) causes tetanus, not an enteric disease.

• Question 842:

  below figure shows a lead II electrocardiogram from a normal 30-year-old female at rest. If a beta- adrenergic agonist such as isoproterenol is administered intravenously to this woman, what will be her resulting heart rate in beats per minute?

  

  A. 40
  B. 55
  C. 65
  D. 70
  E. 90
  E. 90

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  Explanation

  Section: Physiology From inspection of the figure, the R-R interval (time between heart beats) at rest is 0.83 seconds; 60 seconds/minute divided by 0.83 yields a resting heart rate of 72 beats/minute. Since activation of receptors in the sinoatrial node will result in increased heart rate, the only alternative larger than 72 beats/ minute is E., 90 beats/minute.

• Question 843:

  In renal insufficiency, calcium absorption is reduced and leads to increased bone resorption, a condition referred to as renal osteodystrophy. Treatment with which of the following can assist in the amelioration of the symptoms of this condition?

  A. antidiuretic hormone
  B. calcitonin
  C. calcitriol
  D. growth hormone
  E. parathyroid hormone
  C. calcitriol

• Question 844:

  A 32-year-old female professional gardener complains of increasing muscle weakness and fatigue during the day, requiring her to take frequent rests. She also reports that she cannot enjoy her meals any more because her muscles of mastication quickly weaken and she has to stop chewing. When she watches television at night for a long period of time, her vision becomes blurry and she sees double. Her neurologist makes a preliminary diagnosis of myasthenia gravis. Which of the following is the cause of myasthenia gravis?

  A. Acetylcholine synthesis in motor neurons is impaired.
  B. Acetylcholinesterase synthesis is inhibited.
  C. Autoantibodies destroy cholinergic receptors at the postsynaptic membrane preventing binding of acetylcholine.
  D. Neurotransmitter release is impaired at the presynaptic membrane of the neuromuscular junction.
  E. Signal transduction within the muscle is impaired.
  C. Autoantibodies destroy cholinergic receptors at the postsynaptic membrane preventing binding of acetylcholine.

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  Explanation

  Section: Anatomy Myasthenia gravis is an autoimmune disorder where autoantibodies target the postsynaptic cholinergic receptors and destroy them. Acetylcholine released from motor neurons is thus unable to bind and the muscle contraction weakens due to decreased neurotransmitter communication. Acetylcholine synthesis in motor neurons (choice A) remains normal. Acetylcholinesterase (choice B) is the enzyme which degrades acetylcholine and its synthesis is not affected in myasthenia gravis. The cholinergic neurotransmitter release mechanism (choice D) at the presynaptic membrane, as well as the signal transduction mechanism (choice E) within the muscle, remain normal.

• Question 845:

  The structure indicated by arrow 2 in following figure, is which of the following?

  

  A. ethmoidal sinus
  B. inferior nasal meatus
  C. infratemporal fossa
  D. maxillary sinus
  E. sphenoidal sinus
  A. ethmoidal sinus

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  Explanation

  Section: Anatomy This axial scan is at the level of the orbits as indicated by the insert at the bottom right and the eyeballs in the orbits. Arrow 2 points to the ethmoidal sinus located medial to the orbits. The sinus is divided into compartments by the air cells. The maxillary sinus (choice D) and the inferior nasal meatus (choice B) are located inferior to the level of this scan and are not seen. The sphenoidal sinus (choice E) is indicated by arrow 3 and the infratemporal fossa (choice C) by arrow 1.

• Question 846:

  A professional football player was diving for a touchdown when his face mask was grabbed and wrenched, causing neck hyperextension and rotation to the right. When brought to the sideline, the player complained of a burning sensation radiating down the right upper extremity and neurological examination revealed right lateral weakness of this limb. Movements affected were arm rotation and flexion, elbow flexion, forearm supination, and thumb flexion. The patient is diagnosed with a brachial plexus injury at the level of C6. 29. Which of the following muscles can perform arm and elbow flexion along with forearm supination?

  Which of the following innervates the muscle identified?

  A. median
  B. musculocutaneous
  C. radial
  D. suprascapular
  E. ulnar
  B. musculocutaneous

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  Explanation

  The musculocutaneous nerve innervates the biceps brachii along with the coracobrachialis and the brachialis. It continues into the forearm as the lateral antebrachial cutaneous nerve. In this brachial plexus injury, the sudden hyperextension and right rotation of the neck narrow the intervertebral foramina through which exit the cervical nerves. Injury at the level of C6 will affect the musculocutaneous nerve along with the suprascapular nerve, resulting in impairment of the movements described. The median nerve (choice A) and ulnar nerve (choice E) innervate muscles not in the arm but in the forearm and hand. In the arm, the radial nerve (choice C) is the nerve to the only muscle in the extensor compartment, the triceps brachii. The suprascapular nerve (choice D) innervates two muscles of the rotator cuff, the supraspinatus and infraspinatus. It is affected in a brachial plexus injury at the C6 level.

• Question 847:

  A patient has been admitted for hematemesis (vomiting of blood). Endoscopic examination reveals bleeding esophageal varices resulting from portal obstruction. These varices represent anastomoses between branches of which of the following?

  A. inferior vena cava with a patent ductus venosus
  B. left gastric, azygos, and hemiazygos veins
  C. right gastric vein and the inferior vena cava
  D. superior, middle, and inferior rectal veins
  E. veins running on the ligamentum teres and the epigastric veins
  B. left gastric, azygos, and hemiazygos veins

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  Explanation

  Section: Anatomy Obstruction of the portal vein results in an increase in the collateral circulation between veins that normally drain to the portal vein and those that drain to the systemic veins. Choices A, B, D, and E all represent possible collateral venous circulation in case of portal obstruction. Choice Ais rare because the ductus venosus closes after birth. Choice B is correct because varicose veins in this region give rise to esophageal varices. Choice D results in varicose veins in the rectal region. Choice C is incorrect because there is no connection between the right gastric vein and the inferior vena cava. In choice E, enlargement of the epigastric veins results in varicose veins radiating from the umbilicus, the caput medusae.