Discoid eczema — SCE Dermatology MCQ

Correct answer: D — Cutaneous T-cell lymphoma

The correct answer is D, cutaneous T-cell lymphoma (CTCL, most often mycosis fungoides). Discoid (nummular) eczema classically produces coin-shaped, exudative, crusted plaques on the legs with negative mycology, matching this stem, but a single plaque that fails to resolve after four months of adequate potent topical steroid therapy has deviated from the expected clinical course and mandates biopsy to exclude early CTCL. Early mycosis fungoides is well recognised as a great mimicker of eczema and psoriasis, presenting as patches or plaques that are clinically indistinguishable from benign dermatoses and that may transiently improve with topical steroids, delaying diagnosis for years. UK dermatology referral pathways for discoid eczema explicitly state that failure to respond to optimum topical therapy, including superpotent corticosteroids, is a referral trigger for secondary care assessment. Given the malignant potential and the poorer prognosis with delayed diagnosis, any solitary, treatment-refractory eczematous plaque in an older adult must prompt histological exclusion of CTCL. Why the other options are wrong: A. Tinea corporis: already excluded by negative KOH microscopy, and tinea would be expected to improve with antifungal rather than persist despite eczema-directed topical therapy. E. Chronic plaque psoriasis: psoriasis produces well-demarcated plaques with silvery scale rather than oozing, crusted, weeping lesions, and does not specifically explain treatment-resistant morphology raising malignancy concern in this age group. B. Miliaria rubra: this is an acute, self-limiting eruption caused by sweat duct occlusion in hot, humid conditions, presenting as small vesicular papules rather than persistent coin-shaped plaques over months. C. Molluscum contagiosum: presents as discrete umbilicated papules, not confluent itchy eczematous plaques, and is not a relevant differential for a lesion unresponsive to topical steroids over four months. Key point: any solitary discoid eczema-like plaque that fails to respond to adequate, appropriately potent topical therapy must be biopsied to exclude cutaneous T-cell lymphoma.

Reference: Right Decisions Scotland / Centre for Sustainable Delivery (adapted from British Association of Dermatologists Referral Guidelines), Nummular/Discoid Eczema guidance: refer to dermatology if not responding to optimum topical therapy including superpotent topical corticosteroids. https://www.rightdecisions.scot.nhs.uk/borders-ref-help-toolkit/dermatology/cfsd-nummular-discoid-eczema/