Serotonin syndrome — MRCPsych Paper A MCQ

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EasyPsychopharmacologySerotonin syndromeMRCPsych Paper A

A 45-year-old woman taking venlafaxine is prescribed tramadol for back pain. She presents with agitation, diarrhoea, hyperreflexia and inducible clonus. What is the most likely explanation?

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Correct answer: B — Serotonin toxicity due to combined serotonergic drugs

Explanation lettering: E = shown as A · A = shown as B · D = shown as C · B = shown as D · C = shown as E

The correct answer is A. Venlafaxine is an SNRI and tramadol has additional serotonergic (serotonin/noradrenaline reuptake-inhibiting) activity; the combination is a well-recognised precipitant of serotonin toxicity, and both UK SmPCs warn against it. A serotonergic drug exposure with inducible clonus, agitation, hyperreflexia and diarrhoea satisfies the Hunter criteria, in which spontaneous or inducible clonus is the most specific neuromuscular sign. Neuroleptic malignant syndrome (D) requires dopamine-antagonist exposure and typically develops over days with lead-pipe rigidity and hyporeflexia, not clonus. Lithium toxicity (C) requires lithium exposure and causes coarse tremor, ataxia and confusion. Anticholinergic toxicity (E) produces dry skin, mydriasis, urinary retention and reduced bowel sounds—the opposite of diarrhoea—without clonus. Alcohol withdrawal (B) needs a compatible drinking history and causes tremor and autonomic arousal rather than hyperreflexia with inducible clonus. Always reconcile analgesic prescriptions in patients taking antidepressants.

Reference: Electronic Medicines Compendium (eMC). Tramadol 50 mg Capsules, hard — Summary of Product Characteristics, sections 4.4/4.5 (serotonin syndrome with SSRIs/SNRIs). 2025. https://www.medicines.org.uk/emc/product/14961/smpc