Lamotrigine Rash — MRCPsych Paper B MCQ

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ModerateMood DisordersLamotrigine RashMRCPsych Paper B

A 45-year-old woman with bipolar I disorder has taken lithium and quetiapine at unchanged doses for 2 years. Four weeks ago, lamotrigine was added, and its dose was increased from 25 mg daily to 100 mg daily over 10 days. She now presents with fever and a widespread tender erythematous rash with oral mucosal erosions. Which is the most likely cause?

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Correct answer: D — Lamotrigine

The correct answer is D, lamotrigine. Recent initiation, faster-than-recommended dose escalation, fever, a widespread tender rash and mucosal erosions indicate a severe cutaneous adverse reaction such as Stevens–Johnson syndrome or toxic epidermal necrolysis. These reactions occur most often during the first eight weeks of lamotrigine treatment, and risk increases when recommended titration is exceeded. Lamotrigine should be stopped immediately and the patient requires urgent hospital assessment. Lithium can cause rashes and DRESS, while quetiapine has very rarely caused severe cutaneous reactions; however, both have been unchanged for 2 years. Their chronology therefore makes them substantially less likely. A viral exanthem is less consistent with the mucosal erosions and high-risk drug exposure.

Reference: Electronic Medicines Compendium, Lamotrigine Mylan 25 mg Dispersible Tablets, Summary of Product Characteristics, sections 4.2 and 4.4, updated 11 August 2026. https://www.medicines.org.uk/emc/product/2567/smpc