Pityriasis versicolor (tinea versicolor), a fungal infection producing a rash on the skin of a woman's back. Pityriasis versicolor is caused by a yeast, Malassezia furfur, which is believed to be the parasitic form of a common saprophytic yeast Pityrosporum orbiculaire, which may be found on normal skin. Although the infection is most common in tropical countries, it has a world-wide distribution. Climatic factors or exposure to sunlight are believed to trigger the infection in many cases.