A 24-year-old man is brought to the emergency department after 2 days of profuse watery diarrhea and poor oral intake. He reports dizziness when standing and decreased urine output. Vital signs: T 37.0°C (98.6°F), HR 118/min, BP 92/56 mm Hg, RR 16/min. Physical exam shows dry mucous membranes and delayed capillary refill. Labs: serum Na+ 147 mEq/L, K+ 4.2 mEq/L, BUN 48 mg/dL, creatinine 1.6 mg/dL. Urinalysis is otherwise unremarkable. Which of the following additional findings is most consistent with the mechanism of his acute kidney injury?