Contact Dermatitis — SCE Dermatology MCQ
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Correct answer: D — Azo dyes in textiles, sulfonamides, and local anaesthetic esters (benzocaine)
The correct answer is D, azo dyes in textiles, sulfonamides, and local anaesthetic esters (benzocaine). Para-phenylenediamine (PPD) is a potent para-amino sensitiser, and patients allergic to it show well documented cross-reactivity with other chemicals sharing the same para-amino benzene ring structure. This cross-reaction group includes azo and aniline dyes used in textiles, topical and systemic sulfonamides, and ester-type local anaesthetics such as benzocaine and procaine, so patients with confirmed PPD allergy on patch testing need advice to avoid all of these, not just further hair dye exposure. This is the basis of para-group cross-reactivity taught in UK contact dermatitis and patch test clinics, and is central to patient counselling after a positive PPD patch test.
Reference: https://www.nice.org.uk/guidance/conditions-and-diseases/skin-conditions