[A case of black dot ringworm on the right forearm caused by Trichophyton tonsurans].

We report a case of black dot ringworm on the right forearm caused by Trichophyton (T.) tonsurans. A 16-year-old male high school Judo-wrestler visited our clinic on September 30, 2005, complaining of a round erythematosquamous eruption with a distinct margin on the right forearm. Black dots were observed in the lesion. KOH-Parker ink prepared direct microscopy revealed abundant large endotrix arthroconidia in the hair shaft. Culture yielded yellowish-brown colonies. The isolate produced numerous round, short club-shaped microconidia along the hyphae unstained with lactophenol cotton blue (resembling matchsticks) and chlamydospores. PCR-RFLP analysis of internal transcribed spacer regions of ribosomal DNA revealed a banding pattern compatible with T. tonsurans. The lesion was cured by daily administration of 125 mg of terbinafine for 13 weeks.