uk-dgm MCQ (SBA): ACE Inhibitor and Lithium Interaction

A 67-year-old man with a history of bipolar affective disorder controlled on Lithium presents with confusion, coarse tremor, and diarrhoea. He has recently been started on Ramipril for hypertension. His blood tests show: Lithium level 1.8 mmol/L (therapeutic range 0.4-1.0), Na 140, K 5.1, Creatinine 152 µmol/L. What is the MOST likely cause of his Lithium toxicity?