Pityriasis Rubra Pilaris — SCE Dermatology MCQ

Correct answer: E — Pityriasis rubra pilaris (PRP)

The correct answer is E, Pityriasis rubra pilaris (PRP). PRP characteristically produces orange-red, salmon-coloured scaly plaques that spread along Blaschko-like distribution and skin cleavage lines, with follicular keratotic papules coalescing into larger plaques while leaving discrete unaffected patches, the pathognomonic 'islands of sparing'. Histologically, PRP shows a checkerboard pattern of alternating orthokeratosis and parakeratosis in both vertical and horizontal planes, together with thick suprapapillary plates, follicular plugging and a sparse superficial dermal infiltrate, features that reliably separate it from psoriasis. This combination of orange-red colour, islands of sparing and checkerboard hyperkeratosis is considered the clinical and histological hallmark of PRP and is not reproduced by the other papulosquamous disorders listed. Why the other options are wrong: D. Eczema: presents with ill-defined erythematous, vesicular or lichenified plaques with spongiosis on histology, not orange-red plaques with islands of sparing or checkerboard hyperkeratosis. A. Psoriasis: shows well-demarcated salmon-pink plaques with silvery scale, but histology reveals confluent parakeratosis, regular acanthosis with elongated rete ridges and Munro microabscesses, not an alternating orthokeratosis/parakeratosis checkerboard, and islands of sparing are not a feature. C. Drug eruption: typically produces a more uniform morphilliform or urticated eruption temporally linked to a new medication, with histology showing interface dermatitis or eosinophils, without follicular islands of sparing or checkerboard keratinisation. B. Mycosis fungoides: presents with patches or plaques that may mimic eczema or psoriasis, but biopsy shows atypical lymphocytes with epidermotropism and Pautrier microabscesses, features entirely distinct from the checkerboard keratosis of PRP. Key point: Orange-red plaques with islands of sparing plus checkerboard alternating orthokeratosis/parakeratosis on biopsy is diagnostic of pityriasis rubra pilaris and distinguishes it from psoriasis.

Reference: Primary Care Dermatology Society (PCDS), Pityriasis rubra pilaris, Clinical Guidance, https://www.pcds.org.uk/clinical-guidance/pityriasis-rubra-pilaris