Dermoscopic Examination of Tinea Nigra on the Plantar Foot Unveils a Parallel Ridge Pattern Mimicking Acral-Lentiginous Melanoma

A man in his thirties patient visited his previous physician with the chief complaint of a pigmented macule on the sole of his left foot that had steadily grown over the past two years. A dermoscopic examination of the lesion showed a parallel ridge pattern (PRP); therefore, acral-lentiginous melanoma (ALM) was suspected, and the patient was referred to our hospital for a more detailed examination. On closer inspection, PRP had a well-defined and irregular border with no variation in color, which is atypical for ALM which is characterized by asymmetrical structure and variation in colors. Therefore, a skin biopsy was performed for a definite diagnosis, and the histopathological findings showed mycelia in the horny layers. Fungal culture revealed Hortaea werneckii, and the patient was diagnosed with tinea nigra (TN). Although TN is a rare fungal infection in Japan, we emphasize that this fungal infection should be noted as a mimic of ALM and should be considered when PRP is detected on the palms and soles by dermoscopic examination.