Why does tinea versicolor cause hypopigmented spots on dark skin?

Tinea versicolor, also known as pityriasis versicolor, is a common superficial yeast infection caused by the lipophilic organism Malassezia furfur and other species in this genus. The typical presentation in darker skin types is that of multiple hypopigmented thin plaques with fine scale distributed over the densely seborrheic skin of the chest, upper back, and proximal upper extremities. The cause of the hypopigmentation is not completely understood; however, extracts from cultured organisms contain dicarboxylic acids that may competitively inhibit tyrosinase, an enzyme important in melanin production. Production of other indoles or tryptophan-based metabolites may also be involved in the resultant hypopigmentation.

Thoma W, Kramer HJ, Mayser P: Pityriasis versicolor alba, J Eur Acad Dermatol Venereol 19:147–152, 2005.