Pityriasis rotunda.

A 42-year-old man presented with asymptomatic, sharply-demarcated, round, scaly lesions on his forearms that had been present for several months. A skin biopsy specimen was consistent with pityriasis rotunda. Pityriasis rotunda is a disorder of keratinization, which is thought to be a form of acquired ichthyosis, a delayed presentation of congenital ichthyosis, or a cutaneous manifestation of systemic disease. Patients with pityriasis rotunda may be classified into one of two groups, which are based on ethnicity, number of lesions, family history, and association with systemic diseases. Treatment is challenging, but the use of lactic acid lotion and oral vitamin A has shown some promise.