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Photoallergic Dermatitis — SCE Dermatology MCQ

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HardPhotobiologyPhotoallergic DermatitisSCE Dermatology

A 30-year-old man presents with an itchy rash in a photodistributed pattern on his forearms and neck. He works outdoors as a builder. He has been applying a topical NSAID (Ketoprofen gel) to his shoulder for a sports injury. The eruption developed 48 hours after sun exposure. What is the most likely diagnosis?

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Correct answer: C — Photoallergic contact dermatitis to topical Ketoprofen

The best answer is “Photoallergic contact dermatitis to topical Ketoprofen”. 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 “Drug-induced phototoxicity, after clinicopathological correlation”, “Polymorphic light eruption, after specialist assessment”, “Contact dermatitis from cement”, “Sunburn, within an appropriate UK pathway” 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