Focal Pigmented Squamous Cell Carcinoma (pSCC) In Situ of the Nail: A Rare Case Emphasizing Diagnostic Vigilance.

Pigmented squamous cell carcinoma (pSCC) of the nail unit is an uncommon condition that can mimic benign entities. Clinically, pSCC can present as longitudinal melanonychia, onycholysis, or verrucous plaques, potentially leading to diagnostic delays. The pigmented variant is particularly challenging to diagnose, as it may be mistaken for benign melanocytic lesions. Histologically, pSCC demonstrates atypical keratinocyte proliferation with pigmentation, and immunohistochemical analysis is crucial to exclude melanocytic involvement. This case report describes a 25-year-old immunocompetent female with Fitzpatrick type IV skin who presented with progressive nail discoloration and sensitivity of the right thumb, persisting for approximately one year. Despite the absence of a personal or family history of skin malignancy, clinical evaluation and subsequent biopsy revealed focal pSCC in situ with wart-like histopathological features, including full-thickness epithelial atypia, papillomatosis, hypergranulosis, and keratohyalin granule formation. The patient was referred to Moh's micrographic surgery. This case underscores the importance of maintaining a broad differential diagnosis when evaluating persistent or evolving nail pigmentation, especially in patients without classic risk factors. The wart-associated histological changes suggest a possible link to human papillomavirus (HPV) infection, warranting further investigation into viral oncogenesis in nail unit SCC. Clinicians should maintain a high index of suspicion for atypical presentations, particularly in cases with progressive changes and associated pain, to ensure timely diagnosis and optimal management.