Polymorphic Light Eruption — SCE Dermatology MCQ

Correct answer: A — Polymorphic light eruption

Polymorphic light eruption (PLE), option A, is the correct diagnosis. It is the most common idiopathic photodermatosis in the UK, presenting with a pruritic, polymorphic eruption (papules, papulovesicles, plaques) confined to sun-exposed skin, appearing hours after UV exposure and settling within days once sun exposure stops. Although the morphology varies between patients, it is characteristically monomorphic within the same individual on repeated episodes. The classic springtime pattern reflects relative loss of UV tolerance built up the previous summer, with re-exposure in spring triggering the reaction before a 'hardening' effect develops later in the season. The clinical picture, seasonality and rapid resolution with avoidance, without systemic features, are diagnostic of PLE and do not require biopsy or serology in typical cases. Why the other options are wrong: B. Drug-induced photosensitivity: this requires a causative photosensitising drug (eg thiazides, tetracyclines, amiodarone) and typically produces an exaggerated sunburn-like or phototoxic reaction rather than a recurrent seasonal polymorphic eruption; no drug history is given here. C. Chronic actinic dermatitis: occurs mainly in older men, is persistent and eczematous rather than seasonal and self-limiting, and is associated with an abnormally low minimum erythema dose (MED) on photo testing, unlike PLE where MED is normal. E. Lupus erythematosus: cutaneous lupus causes photosensitive but usually fixed, scarring or discoid lesions with systemic or serological markers (ANA, anti-dsDNA) and lacks the rapid days-to-resolve, recurrent spring pattern described. D. Solar urticaria: onset is within minutes of UV exposure and lesions are urticarial wheals resolving within hours, not the delayed (2 to 6 hour), polymorphic, days-lasting eruption described. Key point: A delayed (hours) onset, polymorphic but individually consistent eruption on sun-exposed skin that recurs each spring and resolves within days is the hallmark of polymorphic light eruption.

Reference: British Association of Dermatologists, Patient Information Leaflet: Polymorphic Light Eruption, skinhealthinfo.org.uk, https://www.skinhealthinfo.org.uk/condition/polymorphic-light-eruption/