Kaplan USMLE Step 1: Woman with chest pain and elevated cardiac enzymes

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.

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.

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.