A 70-year old man presented with 20-30 pruritic hyperpigmented, flat papules and annular plaques with squamous borders in the perianal region for 20 years, unresponsive to topical corticosteroids. Histological examination revealed cornoid lamellae, underlying dyskeratosis and a sparse perivascular lymphocytic infiltrate. The diagnosis porokeratosis ptychotropica was made.
Jans et al. (Sun,) studied this question.