A 72-year-old man with chronic kidney disease stage 3b (baseline creatinine 2.0 mg/dL) undergoes elective coronary angiography for stable angina. He receives 150 mL of iodinated contrast medium. On postoperative day 2, his serum creatinine rises to 3.8 mg/dL, and urine output is 1.2 mL/kg/hr. Urine dipstick shows no proteinuria, and microscopy reveals muddy brown casts. What is the most likely diagnosis?