A patient presents with salmon-coloured plaques with islands of normal skin (skip areas) on the trunk and erythroderma. Palmoplantar keratoderma is present. Follicular keratotic papules (nutmeg-grater texture) are seen on the dorsa of fingers. This is type I classic adult pityriasis rubra pilaris (PRP). The key histological feature that differentiates PRP from psoriasis is:

• A Presence of Munro microabscesses
• B Psoriasiform acanthosis with suprapapillary thinning
• C Lichenoid infiltrate at the DEJ
• D Alternating orthokeratosis and parakeratosis in a checkerboard pattern with follicular plugging ✓

Explanation

The pathognomonic histological feature of pityriasis rubra pilaris (PRP) is the alternating orthokeratosis and parakeratosis in a checkerboard (horizontal and vertical) pattern within the stratum corneum, combined with follicular infundibular hyperkeratosis and acanthosis. Munro microabscesses and suprapapillary thinning characterise psoriasis. The checkerboard parakeratosis pattern on histology, combined with the clinical triad (follicular keratotic papules, PPK, skip areas/islands of sparing), distinguishes PRP from psoriasis.

Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.