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(also called tinea versicolor), a fungal disease of the skin that proceeds without affecting the hair or nails. Pityriasis versicolor is classified as a dermatomycosis. The causative agent is the pathogenic mycelial fungus Microspo-ron furfur. The disease is localized predominantly on the skin of the chest, back, neck, and shoulders. It is characterized by the appearance of sharply delineated, gradually enlarging, irregularly shaped desquamatory scales that are yellow-pink, light brown, or dark brown in color. As a rule, no internal sensations arise. Without proper treatment, the disease, which is only slightly infectious, may persist for decades. Depigmented spots mark the sites of previous rashes, especially after exposure to ultraviolet radiation. Pityriasis versicolor is treated with desquamatory agents and can be prevented with conscientious care of the skin and control of perspiration.