A 67-year-old man is admitted to the ICU for septic shock due to pneumonia. He requires norepinephrine and has received multiple doses of broad-spectrum antibiotics. Over the next 48 hours, his urine output decreases and his serum creatinine rises from 1.0 mg/dL to 3.1 mg/dL. Urinalysis shows granular (“muddy brown”) casts. Laboratory studies show: urine Na+ 55 mEq/L, urine osmolality 290 mOsm/kg, and fractional excretion of sodium (FENa) 3.2%. Which of the following best explains these findings?

(Assume he is not taking diuretics.)