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Allergic Contact Dermatitis — SCE Dermatology MCQ

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ModerateInflammatory DermatosesAllergic Contact DermatitisSCE Dermatology

A 42-year-old woman develops multiple vesicles in a linear distribution on her left arm 48 hours after gardening. She recalls brushing against a plant. The eruption follows lines where the plant contacted the skin. Patch testing shows a positive reaction to Compositae mix. What type of contact dermatitis is this?

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Correct answer: B — Allergic contact dermatitis to plant allergens

Linear distribution of vesicles following plant contact is characteristic of allergic contact dermatitis to plant allergens. Compositae (Asteraceae) family plants (chrysanthemums, daisies, ragweed, chamomile) contain sesquiterpene lactones, which are potent contact allergens. The positive patch test to Compositae mix confirms type IV hypersensitivity. Phytophotodermatitis (from furocoumarins in plants like giant hogweed, celery) produces a phototoxic reaction that is non-immunological and appears as erythema and hyperpigmentation in sun-exposed plant contact sites. Linear pattern is a key clue for contact with plants.

Reference: BAD 2017 Contact Dermatitis Guidelines