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Id Reaction — SCE Dermatology MCQ

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ModerateCutaneous InfectionId ReactionSCE Dermatology

A 30-year-old man with tinea pedis presents with intensely pruritic vesicular eruption on his hands. His hands show no fungal elements on KOH scraping, and the eruption appeared simultaneously with the foot infection worsening. What is this hand eruption called?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: C — A sterile id reaction, also termed a dermatophytid

Explanation lettering: E = shown as B · B = shown as D · D = shown as E

C is correct. A dermatophytid is a sterile inflammatory id reaction occurring at a site distant from active dermatophyte infection. Vesicular hand lesions can accompany inflammatory tinea pedis, while fungal microscopy from the hand eruption remains negative because organisms are absent there. Scabies would require a compatible distribution and evidence of mite infestation rather than synchrony with worsening tinea. Antifungal contact dermatitis depends on exposure at the affected site or transfer and is not defined by a sterile distant reaction. Pompholyx can produce similar deep vesicles but would be an independent diagnosis and lacks the specific temporal association with active tinea. Secondary syphilis may affect palms but produces a different papulosquamous eruption and does not follow worsening foot dermatophytosis. Treating the primary fungal infection usually allows the dermatophytid to settle.

Reference: DermNet dermatophytide reactions review: https://dermnetnz.org/topics/dermatophytide-reactions