Photoallergic contact dermatitis — SCE Dermatology MCQ

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HardCutaneous allergyPhotoallergic contact dermatitisSCE Dermatology

A 54-year-old gardener develops itchy eczematous plaques on the face, V of the neck and dorsal hands every summer. The eruption is worse after using a new sunscreen and spares skin under his watch strap and collar. Standard patch testing is equivocal, and the eruption improves during a cloudy fortnight. What is the most appropriate investigation?

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Correct answer: D — Photopatch testing to sunscreen filters

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

A is correct. Photopatch testing evaluates suspected photoallergic contact dermatitis by applying duplicate allergen series and irradiating one set with ultraviolet A. A reaction confined to, or stronger at, the irradiated site supports photoallergy. The summer recurrence, exposed-site distribution, sparing beneath clothing and watch strap, sunscreen association and equivocal standard patch test make this the most informative investigation. Wood lamp examination is used for selected infections and pigmentary disorders, not sunscreen photoallergy. Antinuclear antibody testing may contribute when connective-tissue photosensitivity is clinically suspected but is not the first test for this exposure-linked eczematous pattern. Skin-prick testing assesses immediate IgE-mediated allergy rather than delayed photoallergic dermatitis. Dermoscopy may describe surface morphology but cannot identify the causative photoallergen.

Reference: British Photodermatology Group photopatch-testing guidance: https://onlinelibrary.wiley.com/doi/10.1046/j.1365-2133.1997.06073.x