A 70-year-old woman presents with rising creatinine from 80 to 180 µmol/L over 3 days following a bout of diarrhoea. She is taking an ACE inhibitor and a thiazide diuretic. Blood pressure is 95/60 mmHg and she appears clinically dehydrated. Urinalysis shows no blood or protein and microscopy is bland. What is the most likely cause of her acute kidney injury?