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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 351:

  A 48-year-old female patient is brought to the emergency room by her husband. He reports that his wife suffers from hypertension but, as a high-level executive with a lot of pressure at work, she has been neglecting to take her medication. This morning, as he entered the garage to leave for work, he found his wife lying on the ground next to her own car. She was experiencing uncontrolled flailing of the left arm and leg. What is the most likely site of brain lesion in this patient?

  A. anterior limb of the left internal capsule
  B. anterior limb of the right internal capsule
  C. cerebellum
  D. left subthalamic nucleus of Luys
  E. right subthalamic nucleus of Luys
  E. right subthalamic nucleus of Luys

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  Explanation

  Section: Anatomy Hemiballismus is a movement disorder characterized by involuntary large amplitude movements of one or both limbs on one side of the body. It results from infarct damage to the contralateral subthalamic nucleus of Luys, in this case the right one. The left subthalamic nucleus (choice D) controls the limbs on the right side of the body, which are not affected in this case. The anterior limbs of the internal capsule (choices Aand B) contain mainly thalamocortical and corticothalamic fibers and lesions in these areas do not result in hemiballismus. Lesions in the cerebellum (choice C) also do not result in hemiballismus.

• Question 352:

  An elderly patient, known to suffer from dementia, was found walking aimlessly in the fields in hot weather for several hours before being brought to the clinic. Which of the following would be the most useful laboratory test?

  A. electrocardiogram
  B. electroencephalogram
  C. pneumoencephalogram
  D. serum electrolyte levels
  E. urinalysis
  D. serum electrolyte levels

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  Explanation

  Section: Behavioral Science and Biostatics An elderly woman exposed to hot sunlight for hours may be dehydrated, and testing serum electrolyte levels may confirm this suspicion. Although the other tests (choices A, B, C, and E) may be indicated at some point, they are not the most urgent tests in this case.

• Question 353:

  A 60-year-old male with a chronic cough, bloody sputum, and marked weight loss is diagnosed as having tuberculosis. A "serpentine-like" colonial morphology is noted on Lowenstein- Jensen agar. This latter finding is due to which of the following factors?

  A. coagulase
  B. endotoxin
  C. large "slimy" capsule
  D. trehalose-6,6'-dimycolate
  E. Wax D
  D. trehalose-6,6'-dimycolate

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  Explanation

  Section: Microbiology/Immunology Mycobacteria are rich in lipids. These include mycolic acids, waxes, and phosphatides. In the cell, the lipids are largely bound to proteins and polysaccharides and are, to some extent, responsible for acid- fastness. Virulent strains of tubercle bacilli form microscopic "serpentine cords" in which acid-fast bacilli are arranged in parallel chains. Cord formation is correlated with virulence and a "cord factor" (trehalose-6, 6-dimycolate choice D) has been extracted from virulent bacilli. It inhibits leukocyte migration, causes chronic granulomas, and may serve as an immulogical adjuvant. Virulent mycobacteria do not have endotoxin (choice B) as do the enteric bacteria which may cause inflammation. Nor do mycobacteria produce coagulase (choice A). Mycobacteria colonies are usually rough and dry, not "slimy" from a capsule (choice C). Wax D (choice D) is present in mycobacteria, but is not associated with cord factor.

• Question 354:

  below figure shows a gastric parietal cell with selected membrane transporters and channels. What do X, Y, and Z represent in the figure?

  

  

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

• Question 355:

  Which of the following has a triphasic abstinence syndrome, the first phase being the "crash"?

  A. alcohol
  B. barbiturates
  C. cannabis
  D. cocaine
  E. opiates
  D. cocaine

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  Explanation

  Section: Behavioral Science and Biostatics The abstinence syndrome to cocaine is triphasic. Phase 1 is called "crash," phase 2 is a withdrawal phase with protracted dysphoria, and phase 3 is an "extinction" phase that follows the resolution of withdrawal anhedonia. Choices A, B, C, and E do not show this type of triphasic syndrome.

• Question 356:

  If laboratory tests determine that the patient has a general medical condition that underlies the symptoms, which of the following is most likely?

  A. Addison disease
  B. cancer of the pancreas
  C. hyperparathyroidism
  D. hyperthyroidism
  E. hypothyroidism
  D. hyperthyroidism

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  Explanation

  Section: Behavioral Science and Biostatics Hyperthyroidism is often associated with anxiety and panic symptoms. Although these symptoms may also occur with any of the other medical conditions (choices A, B, C, and E), depression and slowed mentation are more common in hypothyroidism, hyperparathyroidism, and Addison disease; cancer of the pancreas is often associated with severe depression.

• Question 357:

  A 66-year-old male presents with angina and dyspnea on exertion. Auscultation of the chest reveals a loud systolic murmur. He is diagnosed with aortic valvular sclerosis. This stenosis of the aortic valve will cause a murmur that can be heard loudest at which interval shown in below figure?

  

  A. between A and B
  B. between B and C
  C. between C and D
  D. between D and A
  E. between D and B
  C. between C and D

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  Explanation

  Section: Physiology Blood passes through the aortic valve, starting when the aortic valve opens at point C until it closes at point D. Hence aortic valve stenosis will cause turbulent blood flow (murmur) between C and D. Between Aand B (choice A) is the interval of diastolic filling when the aortic valve is closed. Between B and C is the period of isovolumetric contraction (choice B) before the aortic valve opens. Between D and Ais the period of isovolumetric relaxation (choice D) after the aortic valve closes. Between D and B the aortic valve is also closed.

• Question 358:

  Which of the following drugs is useful in asthma because it stabilizes mast cells?

  A. albuterol
  B. cromolyn
  C. ipratropium
  D. salmeterol
  E. theophylline
  B. cromolyn

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  Explanation

  Section: Pharmacology Cromolyn is a prophylactic agent for asthma that appears to stabilize mast cells. Albuterol A. is a shorter acting beta-2-selective agonist that is used for acute, not prophylactic, therapy. Ipratropium (choice C) is an antimuscarinic drug. Salmeterol D. has a long duration beta-2-agonist action that appears to have an anti- inflammatory action and is useful in prophylaxis of asthmatic attacks. It has a slow onset of action that makes it useless in treating acute attacks. Theophylline (choice E) is an intermediate duration agent that acts by inhibition of phosphodiesterase or adenosine receptors.

• Question 359:

  The ability of rod cells in the eye to respond to light and transmit that response to the optic nerve requires that the 11-cis form of vitamin A be attached to which of the following proteins?

  A. cGMP phosphodiesterase
  B. Na+ channel
  C. rhodopsin
  D. scotopsin
  E. transducin
  D. scotopsin

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  Explanation

  Section: Biochemistry Both rod and cone cells contain a photoreceptor pigment in their membranes. The photosensitive compound (photoreceptor) of most mammalian eyes is a complex of protein and an aldehyde form of vitamin A. The protein component is a member of the opsin family called scotopsin. The photoreceptor of rod cells is specifically called rhodopsin (choice C) or visual purple, which is a complex between the protein scotopsin and the 11-cis-retinal form of vitamin A. Intracellularly, rhodopsin is coupled to a specific G- protein called transducin (choice E). When the rhodopsin is exposed to light it is bleached releasing the 11- cis-retinal from opsin. Absorption of photons by 11-cis-retinal triggers a series of conformational changes on the way to conversion all-trans-retinal. The release of opsin results in a conformational change in the photoreceptor. This conformational change activates transducin, leading to an increased GTP-binding by the alpha-subunit of transducin. Binding of GTP releases the alpha- subunit from the inhibitory beta- and gamma-subunits. The GTPactivated alpha-subunit in turn activates an associated phosphodiesterase (choice A), an enzyme that hydrolyzes cyclic-GMP (cGMP) to GMP. Cyclic GMP is required to maintain the Na+ channels (choice B) of the rod cell in the open conformation. The drop in cGMP concentration results in complete closure of the Na+ channels.

• Question 360:

  Which of the following statements concerning muscle spindles is correct?

  A. Activation of type Ia sensory fibers from a given spindle leads to inhibition of the muscle in which that spindle is located.
  B. Alpha motoneurons synapse directly with intrafusal muscle fibers.
  C. Each intrafusal fiber is innervated by two different gamma motoneurons.
  D. Only one type of intrafusal muscle fiber (cell) is present in most muscle spindles.
  E. Type Ia sensory fibers from a spindle form direct synaptic contact with alpha motoneurons in the spinal cord.
  E. Type Ia sensory fibers from a spindle form direct synaptic contact with alpha motoneurons in the spinal cord.

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  Explanation

  Section: Anatomy The type Ia sensory fibers from a spindle form direct excitatory synapses with alpha motoneurons. Activation of type Ia sensory fibers (choice A) leads to excitation of the muscle in which that spindle is located. Alpha motoneurons (choice B) synapse with extrafusal muscle fibers, whereas gamma motoneurons synapse with intrafusal muscle fibers. Each intrafusal muscle fiber (choice C) is innervated by only one gamma motoneuron. Each muscle spindle contains a mixture of both nuclear bag and nuclear chain intrafusal fibers, not just one type as indicated in choice D.