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

USMLE USMLE-STEP-1 Online Questions & Answers

• Question 251:

  Which of the following drugs binds to, and prevents disassembly of, microtubules?

  A. busulfan
  B. cisplatin
  C. mercaptopurine
  D. paclitaxel
  E. vinblastine
  D. paclitaxel

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  Explanation

  Section: Pharmacology Paclitaxel (choice D) is a plant alkaloid that interferes with microtubule disassembly. Busulfan (choice A) and cisplatin (choice B) are alkylating agents that interfere with DNAmobilization and replication. Mercaptopurine (choice C) is an antimetabolite that interferes with purine synthesis. Vinblastine (choice E) and vincristine and vinorelbine are mitotic poisons that prevent cancer cell division by interfering with microtubule assembly and thus immobilizing the mitotic spindle apparatus. Their indications and toxicities are quite different from those of paclitaxel.

• Question 252:

  Aperilunate fracture dislocation is a devastating closed injury of the wrist. It usually results from a fall where the weight of the body is transferred onto the wrist. The hand is caught in the hyperextended and ulnar deviated position. The fracture dislocation involves rupture of interosseous ligaments, joints, and ultimately dislocation/fracture of the lunate bone. In the anatomical position, which carpal bone lies directly distal to the lunate?

  A. capitates
  B. hamate
  C. scaphoid
  D. trapezoid
  E. triquetrum
  A. capitates

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  Explanation

  Section: Anatomy The capitate bone lies directly distal to the lunate. The mechanism of perilunate fracture dislocation involves rupture of the radioscaphocapitate and scapholunate interosseous ligaments, dislocation of the capitolunate joint, rupture of the lunotriquetral interosseous ligament, and finally dislocation/rupture of the lunate. In the anatomical position, the hamate (choice B) is the most medial carpal bone, located just distal to the triquetrum. The scaphoid (choice C) lies lateral to the lunate and proximal to the trapezium (choice D), the carpal bone articulating with the thumb. The trapezoid (choice E) is medial to the trapezoid and distal to the scaphoid. The lunate bone (choice C) lies adjacent to the scaphoid in the proximal row of carpals and with the scaphoid articulates with the radius at the radiocarpal or wrist joint. It is not related to the anatomic snuffbox. The pisiform bone (choice D) is a sesamoid bone in the tendon of the flexor carpi ulnaris on the lateral wrist. It is not related to the anatomic snuffbox.

• Question 253:

  Hormone-secreting chromophils in the pars distalis of the adenohypophysis are classified into acidophils and basophils. Which of the following hormones is secreted by the acidophils?

  A. adrenocorticotropin
  B. follicle-stimulating hormone
  C. luteinizing hormone
  D. prolactin
  E. thryrotropin (thyroid-stimulating hormone)
  D. prolactin

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  Explanation

  Section: Anatomy Prolactin is secreted by one of the two types of hormone-secreting acidophils, the mammotrophs. The other types of acidophils are the somatotrophs, which secrete growth hormone (somatotropin). The hormones listed in all the other choices are secreted by the basophils. Adrenocorticotropin (choice A) is secreted by the corticotrophs. The gonadotrophs secrete either follicle-stimulating hormone (choice B) or luteinizing hormone (choice C). Thyrotropin (choice E) is secreted by thyrotrophs.

• Question 254:

  As part of a neurological examination, a patient is placed in a chair that rotates to the right under the control of a computer so that the patient's head and body move in unison with the chair. How would a healthy person react immediately upon getting off the chair that turned for 1 minute?

  A. behave the same as before the rotation
  B. display post-rotational eye nystagmus to the right
  C. experience a vertigo that would be diminished if he immediately got back in the rotating chair
  D. experience "past pointing" to the left when asked to reach a target
  E. fall to the left when asked to walk on a straight line
  C. experience a vertigo that would be diminished if he immediately got back in the rotating chair

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  Explanation

  Section: Physiology For about 2030 seconds after the rotation stops, the endolymph of the vestibular system continues to rotate due to its inertia. The brain still receives the input of rotation, while the input from the person's eyes and mechanoreceptors signal that the motion stopped. This conflicting information causes dizziness which would disappear if the person continue rotating. The eyes of a healthy person would be "flicking" in the direction opposite to the direction of rotation, so that nystagmus would be to the left, not to the right (choice B). If the person attempts to reach out and touch a target during the postrotatory nystagmus, the movement will miss the target in the direction of the slow phase of nystagmus, to the right in this case, not to the left (choice D). An attempt to walk during the period of nystagmus and vertigo induced by ceasing rotation often leads to a fall, in this case to the right, not to the left (choice E). The compensatory movements to the right are due to the fact that the brain interprets the continuing rotation of the endolymph in relation to the now unmoving outer walls of the vestibular organs as if a rotation to the left has begun. This incorrect input signal is processed by the brain to influence eyes, trunk, and limb muscles to exert compensatory actions. The responses are entirely normal and their absence could indicate dysfunction of the peripheral or central vestibular system (choice A).

• Question 255:

  Your patient presents in your office complaining of hoarseness. During your examination, you find that one vocal fold has deviated toward the midline and does not abduct during deep inspiration or vocalization. You also observe that touch sensation in the vestibule of the larynx appears to be intact.

  You suspect that a nerve has been damaged, but which nerve is most likely involved?

  A. external laryngeal
  B. glossopharyngeal
  C. inferior laryngeal
  D. internal laryngeal
  E. superior laryngeal
  C. inferior laryngeal

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  Explanation

  Section: Anatomy The inferior laryngeal nerve is motor to the intrinsic muscles of the larynx except the cricothyroid muscle, and is sensory to the larynx below the level of the vocal fold. The external laryngeal nerve (choice A) is the branch of the superior laryngeal nerve that contains the motor fibers to the cricothyroid muscle and to the cricopharyngeus muscle. The glossopharyngeal nerve (choice B) does not supply either motor or sensory fibers to the larynx. The internal laryngeal nerve (choice D) is the branch of the superior laryngeal nerve that contains the sensory fibers to the laryngeal mucosa above the vocal fold. The superior laryngeal nerve (choice E) is sensory to the laryngeal mucosa above the vocal fold and also includes motor fibers to the cricothyroid muscle.

• Question 256:

  Which of the following statements about the leukotrienes LT LT and LT is most correct?

  

  A. Their biosynthesis is inhibited by aspirin.
  B. They are chemotactic agents for polymorphonuclear leukocytes.
  C. They are potent bronchoconstrictor substances.
  D. They are synthesized and stored in platelet granules.
  E. They have few cardiovascular effects.
  C. They are potent bronchoconstrictor substances.

• Question 257:

  An individual harboring a mutation in ornithine transcarbamoylase (OTC) would be expected to exhibit which of the following measurable abnormalities?

  A. citrullinemia
  B. elevated urinary excretion of argininosuccinic acid
  C. elevation in blood orotic acid
  D. excess production of foam cells
  E. uric acid deposition in the joints
  C. elevation in blood orotic acid

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  Explanation

  Section: Biochemistry A deficiency in OTC results in one of the urea cycle defect diseases, which are the major causes of hyperammonemia in the newborn. Differentiation of which urea cycle enzyme is defective and causing the hyperammonemia can be accomplished by analysis of the levels of the various intermediates in the cycle (see below figure for UCD DDx).

  

  Adefect in OTC prevents incorporation of carbamoylphosphate into ornithine (see below figure). The carbamoylphosphate will diffuse out of the mitochondria and serve as a precursor for the synthesis of the pyrimidine nucleotides in excess of need leading to an elevation of orotic acid (pyrimidine intermediate) in the blood. A deficiency of the urea cycle enzyme, ASD, will lead to citrullinemia (choice A). Elevation in excretion of argininosuccinate (choice B) will result from a deficiency in the urea cycle enzyme, arginosuccinase. Foam cells are lipidladen cells that are characteristic of disorders in lipid etabolism such as that seen in NPD, which is caused by a deficiency in sphingomyelinase. Uric acid deposition (choice E) results from excess catabolism of the purine nucleotides.

• Question 258:

  Lack of oxygen (hypoxia) will cause reflex vasoconstriction in the circulation supplying which of the following organs?

  A. brain
  B. heart muscle
  C. kidney
  D. lungs
  E. skeletal muscle
  D. lungs

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  Explanation

  Section: Physiology Local hypoxia causes vasoconstriction in vessels of the pulmonary circulation. This is an adaptive response, since it will shunt blood away from poorly ventilated alveoli. This response differentiates the pulmonary circulation from the vasculature of other organs. In heart muscle, skeletal muscle, kidney, and brain (choices A, B, C, and E) vasodilation occurs in response to local hypoxia.

• Question 259:

  Which of the following drugs blocks the action of angiotensin II at the AT1 receptor?

  A. bradykinin
  B. enalapril
  C. losartan
  D. ondansetron
  E. prazosin
  C. losartan

• Question 260:

  Which of the following persons will have a negative free-water clearance?

  A. one who begins excreting large volumes of urine with an osmolality of 100 mosm/kg H2O after severe head injury
  B. one who drinks 2 L of distilled water in 30 minutes
  C. one who is receiving lithium treatment for depression, and who develops polyuria that is unresponsive to administration ADH
  D. one with an oat cell carcinoma of the lung, who excretes urine with an osmolality of 1000 mosm/kg H2O
  E. one with neurogenic diabetes insipidus
  D. one with an oat cell carcinoma of the lung, who excretes urine with an osmolality of 1000 mosm/kg H2O

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  Explanation

  Section: Physiology A negative free-water clearance occurs when the kidney is reabsorbing more water than it is excreting into the urine and is regulated by ADH. Choice D is correct because lung cancer can cause exogenous production of ADH, which will stimulate increased renal water reabsorption and thus a negative freewater clearance. Choice A is incorrect because the decreased ADH secretion that may occur following a severe head injury will result in a positive free-water clearance. Choice B is incorrect because rapid intake of 2 L of water will inhibit ADH secretion and thus trigger a positive free-water clearance. Choice C is incorrect because the insensitivity to ADH that occurs during lithium treatment will increase water excretion and thus cause a positive free-water clearance. Choice E is incorrect because a person with neurogenic diabetes insipidus has a reduced secretion of ADH, which will cause reduced renal water reabsorption and thus a positive free-water clearance.