ACE-Lithium — MRCPsych Paper B MCQ

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ModerateMood DisordersACE-LithiumMRCPsych Paper B

A patient with bipolar disorder has been stable on lithium. A newly prescribed medicine causes the plasma lithium concentration to rise by reducing lithium excretion through inhibition of the renin–angiotensin–aldosterone system. Which medicine is responsible?

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Correct answer: C — Ramipril

Ramipril is correct. It is an ACE inhibitor acting on the renin–angiotensin–aldosterone system. ACE inhibitors may reduce renal lithium excretion, causing plasma lithium concentrations to rise and increasing the risk of toxicity; lithium levels therefore require close monitoring when ramipril is introduced. Ibuprofen is the most plausible distractor because NSAIDs can also decrease lithium elimination and raise lithium concentrations. However, their interaction is mediated principally through inhibition of renal prostaglandin synthesis and reduced renal perfusion, not ACE-mediated RAAS inhibition. Paracetamol does not share the clinically important NSAID–lithium interaction. Amoxicillin and omeprazole are not established causes of increased lithium concentrations through RAAS blockade.

Reference: Electronic Medicines Compendium. Ramipril 5 mg capsules, Summary of Product Characteristics, sections 4.5 and 5.1. Revised 7 July 2026. https://www.medicines.org.uk/emc/product/7104/smpc