A 78-year-old man is admitted with confusion, decreased urine output, and vomiting for 2 days. His medications include ramipril 10mg OD, indapamide 2.5mg OD, and ibuprofen 400mg TDS PRN for osteoarthritis (taken regularly recently). Blood tests show: Na+ 132 mmol/L, K+ 6.2 mmol/L, Urea 25 mmol/L, Creatinine 350 μmol/L (baseline 110 μmol/L). What is the most likely cause of his acute kidney injury (AKI)?