Fixed Drug Eruption — SCE Dermatology MCQ

Correct answer: E — Fixed drug eruption

The correct answer is E, fixed drug eruption. This diagnosis is defined by exactly the pattern described: a well demarcated violaceous plaque that recurs at the identical anatomical site every time the causative agent, here co-codamol (via its paracetamol or opioid component), is taken, then heals leaving residual post-inflammatory hyperpigmentation. This site-specific recurrence on re-exposure is the hallmark discriminating feature, reflecting activation of site-restricted, drug-specific CD8+ resident memory T cells in the epidermis and dermo-epidermal junction. Paracetamol is a well recognised trigger, alongside NSAIDs, co-trimoxazole and tetracyclines, so the drug history fits. Management is identification and lifelong avoidance of the culprit drug and cross-reacting agents. Why the other options are wrong: A. Insect bite reaction: papulo-urticarial lesions from bites are typically itchy, clustered or linear, related to exposure/season, and do not show an exact single-site relapse tied to drug ingestion. C. Contact dermatitis: this is eczematous, itchy, and linked to topical allergen or irritant contact, not to systemic drug ingestion, and it does not classically leave a fixed violaceous plaque at one recurring site. B. Chronic plaque psoriasis: presents as persistent, scaly, symmetrical plaques over extensor surfaces that wax and wane with disease activity, not lesions that appear and fully resolve in step with a specific medication. D. Lichen planus: causes multiple pruritic, polygonal, violaceous papules, often with Wickham striae and mucosal involvement, and lesions are not confined to one drug-triggered recurring site. Key point: A violaceous plaque that reappears at the exact same site every time a specific drug is taken, then leaves post-inflammatory hyperpigmentation, is diagnostic of fixed drug eruption.

Reference: DermNet NZ, Fixed drug eruption (2023), https://dermnetnz.org/topics/fixed-drug-eruption