Annular Erythema — SCE Dermatology MCQ

Correct answer: D — Annular erythema with trailing scale — consistent with pityriasis rosea, erythema annulare centrifugum, or secondary syphilis depending on clinical context

The correct answer is D, annular erythema with trailing scale, because this specific morphology, a rim of fine scale just inside (trailing) the advancing erythematous edge rather than on top of it, is the defining feature of a group of reactive annular dermatoses rather than a single fixed diagnosis. In pityriasis rosea the herald patch and subsequent lesions show a peripheral collarette of scale attached at the outer edge and free centrally, matching this description. Erythema annulare centrifugum classically shows lesions that expand centrifugally leaving a delicate trailing scale just behind the leading erythematous border. Secondary syphilis can also produce annular plaques with a similar collarette, so serology and systemic features must be sought when this pattern is seen. The key teaching point is that trailing scale is a morphological descriptor, not a diagnosis, and the final diagnosis depends on distribution, evolution and associated clinical clues. Why the other options are wrong: B. Vesicular: describes fluid filled blisters (as in eczema herpeticum or bullous disorders), not a scale pattern at an annular border. E. Psoriasiform: implies thick, silvery scale adherent across the whole plaque surface, typically on extensor surfaces, not a thin scale confined to the trailing inner edge of an annular lesion. A. Nodular: describes a palpable, deep, solid lesion (e.g. nodular basal cell carcinoma or nodulocystic acne), unrelated to superficial annular scaling. C. Pustular: describes lesions containing visible pus (as in pustular psoriasis or folliculitis), a different primary lesion type entirely from a scaly annular border. Key point: trailing scale just inside an expanding annular erythematous edge is a shared morphological clue for pityriasis rosea, erythema annulare centrifugum and secondary syphilis, so the surrounding clinical context (distribution, herald patch, systemic features, serology) is needed to reach the specific diagnosis.

Reference: Primary Care Dermatology Society (PCDS), Pityriasis Rosea and General Dermatology Diagnostic Tool, pcds.org.uk