A 32-year-old woman is admitted with septic shock secondary to acute pyelonephritis. On arrival, her blood pressure is 78/42 mmHg. She receives aggressive fluid resuscitation and requires norepinephrine infusion to maintain adequate blood pressure. By hospital day 3, she is hemodynamically stable and vasopressors have been discontinued for 12 hours. However, her urine output has decreased to 15 mL/hour despite continued maintenance fluids.
Laboratory studies show:
- Serum creatinine: 4.2 mg/dL (baseline 1.0 mg/dL)
- Blood urea nitrogen: 38 mg/dL
- BUN/creatinine ratio: 9
- Fractional excretion of sodium (FENa): 3.5%
- Urine osmolality: 310 mOsm/kg
Urinalysis reveals muddy brown granular casts and renal tubular epithelial cells. Renal ultrasound shows normal-sized kidneys without hydronephrosis.
Which of the following best explains this patient's acute kidney injury?