Photoallergic Dermatitis — SCE Dermatology MCQ

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HardPhotobiologyPhotoallergic DermatitisSCE Dermatology

A 58-year-old man is referred for a photopatch test. The test involves applying allergens to two sites on the back — one site is irradiated with UVA and the other is kept covered. Readings are taken at 48 and 96 hours. A reaction at the IRRADIATED site only indicates what?

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Correct answer: C — Photoallergic contact dermatitis

The best answer is “Photoallergic contact dermatitis”. A photoallergic reaction requires sensitisation and occurs where a chemical allergen and ultraviolet exposure coincide; photopatch comparison separates photoallergy from ordinary contact allergy. The alternatives “False positive”, “Phototoxic reaction”, “Irritant reaction”, “Allergic contact dermatitis” are clinically adjacent possibilities, but they do not fit the defining morphology, distribution, histopathology, risk signal or management sequence in this stem.

Reference: DermNet specialist photocontact dermatitis reference: https://dermnetnz.org/topics/photocontact-dermatitis