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Drug-Induced Photosensitivity — SCE Dermatology MCQ

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ModeratePhotobiologyDrug-Induced PhotosensitivitySCE Dermatology

A 50-year-old man presents with a photosensitive eruption after starting Hydrochlorothiazide. He develops erythema and blistering on sun-exposed areas 24 hours after sun exposure. This is a phototoxic rather than photoallergic reaction. What is the key distinguishing feature of phototoxicity versus photoallergy?

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Correct answer: B — Phototoxicity can occur on first exposure; photoallergy requires prior sensitisation

Phototoxicity is a non-immunological reaction that can occur on first exposure to the drug plus UV light. It is dose-dependent and resembles exaggerated sunburn, confined to sun-exposed areas. Photoallergy is an immunologically mediated (type IV) delayed hypersensitivity reaction requiring prior sensitisation (usually 1-2 weeks), can spread beyond sun-exposed areas (may become generalised), and is not dose-dependent. Common phototoxic drugs include thiazides, tetracyclines, NSAIDs, and fluoroquinolones. Photo-patch testing differentiates photoallergy from phototoxicity by demonstrating immune sensitisation.

Reference: BAD 2019 Photodermatoses Guidelines