A 32-year-old man presents with oliguria and rising creatinine (from 90 to 260 µmol/L over 48 hours) after several days of profuse diarrhoea and vomiting. His blood pressure is 90/55 mmHg, and examination reveals dry mucous membranes with no peripheral oedema. Urinalysis shows no protein or blood. What is the most likely cause of his acute kidney injury?