Pigmented Squamous Cell Carcinoma Presenting as Longitudinal Melanonychia in a Transplant Recipient

We report the case of a 62-year-old black man who was on a maintenance immunosuppressive regimen that included mycophenolate mofetil and cyclosporine following renal transplantation 9 years prior. He presented to the dermatology department for evaluation of a pigmented longitudinal streak on the left third finger adjacent to the lateral nail fold that had been present for several months. He noted that the streak was increasing in size, and his fingertip had recently become tender. The pigmented band was biopsied, and histopathology showed atypia of the epidermis consistent with pigmented squamous cell carcinoma (pSCC). Although subungual melanoma is the most concerning cause for longitudinal melanonychia, there are a number of other potential causes, including fungal infection, trauma, benign melanocytic lesions, or other cutaneous malignancies. Pigmented squamous cell carcinoma is another potential cause of longitudinal melanonychia and should be included in the differential diagnosis, particularly in individuals with skin of color or those who are immunosuppressed. This article highlights features of the clinical presentation of pSCC presenting as longitudinal melanonychia that mimicked the clinical appearance of subungual malignant melanoma in a renal transplant recipient. A review of pSCC and its associated risk factors also is provided.