Merkel Cell Carcinoma With Divergent Differentiation-Or Something Else? A Triphasic Cutaneous Carcinoma With Lineage Plasticity.

Merkel cell carcinoma (MCC) is a rare, aggressive cutaneous neuroendocrine carcinoma that typically arises in sun-exposed skin of older or immunosuppressed patients. Combined or collision tumors involving MCC and other cutaneous malignancies are uncommon, and tumors exhibiting more than two distinct malignant phenotypes are exceptionally rare. We report a case of a triphenotypic carcinoma in the left upper arm composed of a neuroendocrine carcinoma with Merkel cell features, a poorly differentiated carcinoma with squamous/ductal differentiation, and a sarcomatoid component, each demonstrating distinct morphologic and immunophenotypic profiles. While the findings favor classification as MCC with divergent epithelial and sarcomatoid differentiation, the precise lineage and cell of origin cannot be definitively established, and alternative interpretations are considered and discussed. This unusual case highlights the diagnostic challenge posed by plastic cutaneous carcinomas and underscores the importance of comprehensive histologic evaluation, broad immunohistochemical workup, and multidisciplinary management. We review the relevant literature of cases with comparable histomorphology and discuss the implications for diagnosis, staging, and treatment.