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USMLE STEP1 Exam Questions
Step1 (Page 31 )

Updated On: 31-Mar-2026
Page 31 of 171
PDF with 845 Questions

End-stage chronic renal disease is associated with a glomerular filtration rate (GFR) of less than 15 mL/ min and an increase in which of the following blood values?

  1. calcium
  2. erythropoietin (EPO)
  3. hematocrit
  4. phosphate
  5. 1, 25-(OH vitamin

Answer(s): D

Explanation:

Chronic renal failure reduces phosphate excretion and thus phosphate accumulates in the blood. The renal peritubular cells are the major source of circulating EPO, which stimulates red blood cell production. Hence renal failure will decrease EPO levels (choice C) and will decrease hematocrit (choice E) as well. The kidney is responsible for production of vitamin D3, the active
form of vitamin D, and renal failure also results in decreased active vitamin D (choice E). Secondary to this, the circulating levels of calcium will decrease (choice A).



A very thin 15-year-old female presents with headache, polyuria, and grand mal seizures. She answers the question, if she has had to vomit recently, with "frequently" but that these symptoms "are under control now." Her body mass index is 14.1. Serum glucose, calcium, and potassium are normal. Serum sodium, chloride, and osmolarity are low. As a working diagnosis, which of the following is most likely?

  1. Addison disease
  2. central diabetes insipidus
  3. diabetes mellitus
  4. nephrogenic diabetes insipidus
  5. water intoxication

Answer(s): E

Explanation:

Excessive vomiting can lead to large losses in fluid and electrolytes. Dehydration triggers the sense of thirst. Her age, her extremely low body mass index (normal 18.524.9), and her frequent vomiting all point toward anorexia nervosa. Anorexic people might drink large amounts of water to reach target weight. By drinking a lot of water without adequately replacing electrolyte imbalances, water intoxication can result.
Diluted serum sodium levels can lead to headaches and seizures. Although vomiting, weight loss, and fainting due to low blood pressure could point toward Addison disease, the typical symptoms of hypocortisolism, which include hyperkalemia, hyperosmolality, and hyperpigmentation, are not mentioned and hence make choice A not the best one. In patients with diabetes insipidus, whether caused by a lack of ADH (choice B) or by failure of the kidney to adequately respond to ADH (choice D), serum osmolality is expected to be high. Normal glucose values make uncontrolled diabetes mellitus not the best working diagnosis (choice C).



The ventricular action potential labeled N in below figure represents a normal ventricular action potential. Which of the other action potentials (labeled AE) would be most characteristic of the ventricular cell after administration of a drug that partially blocks slow calcium channels?

  1. A
  2. B
  3. C
  4. D
  5. E

Answer(s): D

Explanation:

The slow calcium channels function in the ventricular myocardial cell to allow the influx of calcium into the cell, which has the dual role of triggering the calcium-mediated contractile mechanism and maintaining the depolarized state after the voltage-gated sodium channels close, soon after the rapid upstroke. Hence, blocking these channels will shorten the plateau phase of the action potential as seen in choice D, while also decreasing the contractile force of the heart. In choice A action potential duration is also shortened, but in a manner that would reflect blockade of the rapid voltagegated sodium channels, not the slow calcium channels. Choice B represents no change in the action potential. Choice C would occur if the drug prolonged the open state of the slow calcium channels. The hyperpolarization of choice E is more likely the result of increased potassium channel currents.



Pupil size is an important indicator of brainstem function. Which of the following results in papillary constriction?

  1. atropine, a blocker of muscarinic receptors
  2. decreased parasympathetic activity of inner eye muscle fibers during darkness
  3. general increased sympathetic tone during emotional excitement
  4. increased sympathetic activity of inner eye muscle fibers during darkness
  5. phentolamine, a blocker of alpha adrenergic receptors

Answer(s): E

Explanation:

Pupil diameter is determined by the balance between sympathetic tone to the radial fibers of the iris and parasympathetic tone to the circular pupillary sphincter muscle. Sympathetic activation will result in pupillary dilation via activation of alpha-adrenergic receptors. Hence, an alpha-adrenergic blocker leads to constriction of the pupil. Parasympathetic activation on the other hand will result in pupil constriction via activation of muscarinic acetylcholine receptors, so that pupil dilation occurs as a result of a muscarinic acetylcholine receptor block (choice A). The same is true for decreased parasympathetic activity during darkness (choice B), or increased sympathetic activity, independent if the initial stimulus is due to emotional excitement or darkness (choices C and D).



For the bithermal caloric test, a measure of vestibular sensitivity, warm or cold water is introduced into the ear. This often causes vertigo and nausea as a side effect as a result of which of the following?

  1. convection currents in endolymph
  2. especially fast flow of endolymph
  3. temporary immobilization of cristae ampullares
  4. temporary immobilization of otolith organs
  5. temporary inhibition of cochlear hair cells

Answer(s): A

Explanation:

Water that is either warmer or cooler than body temperature when introduced into the external auditory meatus sets up convection currents within the endolymph of the inner ear. These currents may result in the movement of the cupula, the gelatinous matrix, which sits atop the crista ampullaris and occludes the lumen of the semicircular canal. Displacement of the cupula results in activation of the vestibular hair cells of the ampullar crista. This inappropriate stimulation of the vestibular system conflicts with sensory information from other sensor organs, including vestibular sensors on the contralateral side, vision, and proprioception, and may in turn result in vertigo and nausea. The existence of the many ducts within the inner ear might imply that the fluids that fill them are flowing through the ear. However, neither endolymph nor perilymph "flow" along their respective compartments in the normal or stimulated vestibular system (choice B). Neither hair cells of cristae ampullares (choice C) nor of otholith organs (choice D) nor of the cochlea (choice E) are temporarily immobilized or inhibited by temperature changes.



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