Fixed drug eruption — SCE Dermatology MCQ

Correct answer: E — Fixed drug eruption

The correct answer is E, fixed drug eruption. A well demarcated, dusky or violaceous patch on the glans developing hours after taking a culprit drug, with recurrence at the exact same anatomical site after repeat exposure and residual post-inflammatory pigmentation, is the classic description of fixed drug eruption. NSAIDs such as ibuprofen are among the most common causes, and the genital and oral mucosae are particularly frequent sites for this reaction. The mechanism is a type IV hypersensitivity response in which drug-specific memory CD8+ T-cells resident at the dermo-epidermal junction are reactivated on re-exposure, driving basal keratinocyte damage and melanin release, which produces the dusky colour and residual pigmentation. The temporal link to a specific drug and site-specific recurrence are the key diagnostic clues here, not seen in the other options. Why the other options are wrong: D. Herpes simplex infection: presents with grouped painful vesicles or erosions rather than a single dusky patch, and although it can recur at the same site, there is no drug trigger and systemic prodrome or grouped vesicles would be expected. B. Primary syphilis: produces a painless indurated ulcer (chancre) that appears weeks after sexual exposure, not hours after a tablet, and does not recur at the same site with drug rechallenge. A. Zoon balanitis: causes a chronic, persistent, glazed orange-red patch on the glans in uncircumcised men, evolving over weeks to months, not an acute dusky lesion tied to drug timing. C. Erosive lichen planus: causes chronic erosive or reticulate white lesions with a relapsing but not drug-triggered course, and lacks the sharp temporal relationship to a specific medication. Key point: A dusky, sharply demarcated patch that recurs at the identical site within hours of re-exposure to the same drug is pathognomonic for fixed drug eruption.

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