A 12-year-old boy, who belonged to a football team, visited a dermatology clinic with an erythematous lesion on his left knee in May 2015. Because the KOH examination performed by the previous doctor did not detect any fungi, topical corticosteroid was applied to the lesion but resulted in no improvement. Therefore, he was referred to our outpatient clinic in June 2015. Physical examination revealed a hyperkeratotic erythematous lesion measuring 6 × 5 cm in diameter on the left knee without black-dot ringworm. We performed a direct KOH examination again for corneocytes in the erythematous area, but no fungus was detected. Meanwhile, the growth of Trichophyton tonsurans (T. tonsurans) was confirmed by a skin-tissue culture including follicular tissue taken from the lesion. Additionally, a polymerase chain reaction analysis of DNA extracts from the cultured fungus showed an exact match for T. tonsurans, and we eventually diagnosed this case as having tinea corporis on the left knee caused by T. tonsurans. Because various clinical manifestations of tinea corporis caused by T. tonsurans have been reported, we must perform thorough examinations to make a definitive diagnosis.
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