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 45-year-old woman is admitted to the hospital because of chest pain and markedly elevated cardiac enzymes. Over the course of her hospitalization, her blood pressure decreases to 85/45 mm Hg and pulse increases to 124/min. Her capillary refill is delayed and her extremities are cool. Her jugular venous pressure is 12 mm Hg. Her urine output decreases. Urine analysis reveals yellow-colored urine, negative for bilirubin, blood and protein. Urine microscopy reveals minimal red cells and white cells and the absence of brown granular casts. A diagnosis is made. Which of the following most likely supports this diagnosis?
A. Fractional Na+ excretion = 0.5%
B. Serum urea nitrogen/serum creatinine ratio = 8
C. Urinary sodium = 50 mEq/L
D. Urine osmolality = 250 mOsmol/kg H2O
E. Urine/serum creatinine = 10
The correct answer is A.
Kaplan Medical explains why
Cardiogenic shock following myocardial infarction (MI) can lead to prerenal acute kidney injury (AKI). In response to decreased renal perfusion from shock, the renin-angiotensin-aldosterone system (RAAS) activates and stimulates aldosterone release. Aldosterone increases the reabsorption of Na+ from the renal tubules which lowers fractional excretion of sodium (FeNa). FeNa is usually <1% in prerenal AKI.
Why the other answers are wrong
Choice B. Serum BUN/creatinine ratio in prerenal AKI is usually >20, due to an antidiuretic hormone (ADH)-mediated increase in passive urea reabsorption from the renal tubules.
