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Anticholinergic Burden in Dementia — MRCPsych Paper B MCQ

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ModerateOld Age PsychiatryAnticholinergic Burden in DementiaMRCPsych Paper B

A 75-year-old man with moderate Alzheimer's disease has been stable on donepezil 10 mg daily. Oral oxybutynin is started for urgency urinary incontinence. Over the following weeks, his family notices increased confusion and reduced day-to-day functioning. Physical examination and investigations identify no infection or metabolic disturbance. Which pharmacological explanation best accounts for the psychiatrist's concern?

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Reveal the answer and explanation

Correct answer: B — Oxybutynin blocks muscarinic signalling, opposing donepezil's cholinergic effect and increasing cognitive anticholinergic burden

Explanation lettering: D = shown as A · A = shown as D

Oxybutynin is an antimuscarinic drug, whereas donepezil inhibits acetylcholinesterase and enhances cholinergic transmission. Their opposing pharmacodynamic actions may reduce the symptomatic benefit of donepezil. Oxybutynin also adds substantial anticholinergic burden and can cause confusion or worsen cognition, particularly in an older person with dementia. Its UK SmPC specifically warns that concomitant use may reduce cholinesterase-inhibitor efficacy, while NICE advises minimising medicines associated with anticholinergic burden in dementia. A is therefore incorrect. C and D propose unsupported CYP-mediated interactions rather than the relevant pharmacodynamic antagonism. E is incorrect because neither combination produces serotonin syndrome through serotonergic enhancement.

Reference: Electronic Medicines Compendium, Oxybutynin Hydrochloride Tablets 5 mg, SmPC sections 4.4, 4.5 and 5.1, revised 2026. https://www.medicines.org.uk/emc/product/14310/smpc