A 72-year-old man presents with reduced urine output and confusion after several days of vomiting and diarrhoea. His medications include ramipril and ibuprofen. Blood pressure is 88/54 mm Hg, heart rate 110 beats/min. Laboratory tests show creatinine 260 µmol/L (baseline 100), urea 20 mmol/L and urine sodium 8 mmol/L. What is the most likely cause of his acute kidney injury?