A 67-year-old man is brought to the emergency department from a nursing facility because of decreased urine output for 1 day. He has had poor oral intake for several days due to a viral illness. Medications include lisinopril and ibuprofen as needed for back pain. Vital signs: T 37.1°C (98.8°F), BP 92/56 mm Hg, HR 112/min, RR 16/min. Physical exam shows dry mucous membranes and delayed capillary refill. A Foley catheter is placed and returns only 15 mL of dark urine. Laboratory studies show:

Serum BUN: 48 mg/dL Serum creatinine: 2.4 mg/dL (baseline 0.9 mg/dL)

Which of the following findings is most characteristic of acute kidney injury in this patient?