BG15 Acral pigmentation of the toe: when dermatophyte infection mimics melanoma

Abstract

Pigmented lesions of the skin present a significant diagnostic challenge, particularly in distinguishing benign conditions from malignant melanoma. Diagnostic uncertainty can affect patients and clinicians alike, potentially leading to misdiagnosis, delayed treatment and unnecessary anxiety. We report the case of a 67-year-old man who presented with a > 10-year history of black pigmentation affecting the right fourth toe. The black pigmentation involved the distal digit and entire nail unit, extending across the interphalangeal joint. The lesion remained stable over time and was asymptomatic. Based on the clinical appearance, acral lentiginous melanoma was initially suspected. An initial incisional biopsy demonstrated an increase in basal melanocytes with no histological features of melanoma. Following multidisciplinary team discussion, a repeat incisional biopsy was done. Histology showed squamous papilloma, with no evidence of malignancy. Fungal culture grew Trichophyton rubrum. This fungal organism has been reported as a cause of pigmented onychomycosis, a rare presentation of a dermatophyte infection. The patient was commenced on systemic antifungal therapy and is awaiting clinical follow-up to assess treatment response. This case highlights the diagnostic difficulty of atypical pigmented lesions in acral sites, where clinical suspicion of melanoma is high but histological features are incongruent. It underscores the importance of maintaining a broad differential diagnosis and considering fungal infection as a potential cause of nail and acral pigmentation once melanoma has been excluded.