Porokeratosis ptychotropica: A differential diagnosis for “psoriasis” of the gluteal region

A 51-year-old man presented with an 8-year history of a well demarcated erythematous scaly plaque on the buttocks, extending into the natal cleft. Initial skin biopsy was reported as showing features of psoriasis. Treatment with topical calcipotriol, acitretin, UVB and intralesional triamcinolone were ineffective. Repeat biopsies taken from two different areas showed neutrophilic collections within the stratum corneum and a psoriasiform pattern of hyperplasia supporting the initial diagnosis of psoriasis.