A 72-year-old man presents with dizziness and decreased urine output after several days of vomiting and diarrhoea. Blood pressure is 90/60 mmHg, pulse 110 beats/min, mucous membranes are dry and his JVP is low. Serum urea is 18 mmol/L and creatinine 160 µmol/L (baseline 85 µmol/L). What is the most likely cause of his acute kidney injury?