Psoriasis — SCE Dermatology MCQ

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ModerateInflammatory DermatosesPsoriasisSCE Dermatology

A 55-year-old man presents with a well-demarcated, thickened, erythematous plaque on his scalp that has not responded to topical corticosteroids or antifungals. Biopsy shows parakeratosis, irregular acanthosis, and a superficial perivascular lymphocytic infiltrate with no atypia. There is thinning of the suprapapillary plate with dilated capillaries in the papillary dermis. What is the diagnosis?

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Correct answer: C — Psoriasis

The correct answer is C, Psoriasis. The clinical picture of a well-demarcated, thickened, erythematous scalp plaque unresponsive to steroids and antifungals, combined with the biopsy triad of parakeratosis, regular (even) acanthosis and a superficial perivascular lymphocytic infiltrate without atypia, is classic for plaque psoriasis. The additional finding of suprapapillary plate thinning with dilated, tortuous capillaries in the dermal papillae is the histological correlate of the Auspitz sign (pinpoint bleeding on scale removal) and is a hallmark feature used to distinguish psoriasis from other papulosquamous disorders. Scalp psoriasis is notoriously refractory to standard topical steroid or antifungal regimens alone, which fits the treatment-resistant history in the stem, and typically requires combination therapy such as calcipotriol/betamethasone or coal tar preparations. Why the other options are wrong: B. Tinea capitis: This is a fungal infection producing scaling, alopecia and often kerion formation, with hyphae seen on PAS/GMS staining, and would improve with topical antifungals rather than fail to respond. A. Seborrhoeic dermatitis: Histology shows spongiosis, follicular plugging and a mixed infiltrate with neutrophils at follicular ostia, not the regular acanthosis and suprapapillary thinning described here. E. Mycosis fungoides: This cutaneous T cell lymphoma shows epidermotropism with atypical lymphocytes forming Pautrier microabscesses; the stem explicitly states no atypia, excluding this diagnosis. D. Lichen simplex chronicus: This reflects chronic rubbing/scratching with compact hyperkeratosis, irregular (not regular) acanthosis and vertical collagen streaking in the papillary dermis, without the vascular and suprapapillary changes described. Key point: Regular acanthosis with suprapapillary plate thinning and dilated papillary dermal capillaries (the Auspitz sign correlate), plus absence of atypia, is diagnostic of psoriasis and separates it from lichen simplex chronicus, seborrhoeic dermatitis and mycosis fungoides.

Reference: NICE Clinical Knowledge Summaries (CKS), Psoriasis: diagnosis and clinical features, https://cks.nice.org.uk/topics/psoriasis/