If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 1 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
A 17-year-old boy is brought to the emergency department after sustaining a severe fracture at the base of his skull while skateboarding. Two weeks later, he develops increased thirst with polyuria (~10 L/day). Temperature is 37.8°C (100.0°F), pulse is 98/min, respirations are 16/min, and blood pressure is 115/65 mm Hg. Fasting serum glucose is 98 mg/dL. Laboratory studies show:
pH: 7.41
PCO2: 45 mm Hg
PO2: 96 mm Hg
Urine: Pale color, negative for ketones
Urine specific gravity: <1.005
Which of the following compounds should most likely be administered?
A. Aldosterone
B. Angiotensin II
C. Calcitonin
D. Insulin
E. Vasopressin
The correct answer is E.
Kaplan Medical explains why
Head trauma is associated with transient central diabetes insipidus (DI), which is characterized by decreased vasopression (antidiuretic hormone; ADH) secretion from the posterior pituitary. Decreased ADH results in reduced aquaporin channel-mediated water reabsorption from the kidneys in the distal convoluted tubule and collecting duct, leading to volume depletion, polyuria, and low urine specific gravity. Vasopressin administration can increase renal reabsorption of water in patients with central DI.
Why the other answers are wrong
Choice A: Vasopressin is an antidiuretic hormone (ADH) analog used in the treatment of central diabetes insipidus (DI). Central DI is caused by a deficiency in vasopressin (antidiuretic hormone; ADH), not aldosterone. Additionally, aldosterone levels are typically already elevated in volume-depleted states such as central DI.
