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Solar urticaria — SCE Dermatology MCQ

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HardPhotodermatologySolar urticariaSCE Dermatology

A woman develops widespread wheals and presyncope within five minutes of sunlight exposure. The eruption clears within an hour indoors. Lupus serology and plasma porphyrins are normal. Which investigation most usefully characterises this photodermatosis?

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Correct answer: B — Monochromator testing for the action spectrum and minimal urticarial dose

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

E is correct. The immediate wheal-and-flare response with rapid resolution is solar urticaria. Specialist phototesting exposes defined areas to different wavelengths and graded doses, allowing the action spectrum and minimal urticarial dose to be determined; extensive exposure can produce systemic mediator effects such as presyncope. Delayed UVB provocation is used for disorders such as polymorphic light eruption, photopatch testing investigates photoallergic contact dermatitis, persistent eczema is not the diagnostic endpoint, and porphyria is assessed biochemically rather than by biopsy of resolved urticaria. Identification of provoking wavelengths can also inform photoprotection and desensitisation planning.

Reference: BAD solar urticaria information: https://www.bad.org.uk/pils/solar-urticaria