A 65-year-old man with type 2 diabetes and hypertension (treated with ramipril and amlodipine) is admitted with pneumonia. His baseline creatinine was 110 µmol/L. On day 3 of admission, his creatinine has risen to 250 µmol/L, and his urine output has been <0.5 mL/kg/hr for the past 8 hours. Which of the following is the most likely cause of his acute kidney injury (AKI)?