Question

A 72-year-old woman presents to the gynecology clinic complaining of severe, persistent vulvar pruritus and a burning sensation for the past 8 months. She notes that the symptoms have not improved despite applying an over-the-counter topical hydrocortisone cream. On physical examination, there is a well-demarcated, erythematous, crusted, and slightly weeping plaque measuring 3.5 cm on the right labium majus. A punch biopsy of the lesion is performed. Light microscopy of the hematoxylin and eosin (H&E) stained section reveals an intact basement membrane with a proliferation of large, atypical cells containing abundant pale-staining cytoplasm and prominent nucleoli. These cells are scattered singly and in small clusters throughout the epidermis, predominantly in the basal and suprabasal layers. Immunohistochemical staining demonstrates that the atypical cells are strongly reactive for cytokeratin 7 (CK7) and carcinoembryonic antigen (CEA), but are completely negative for S100 protein, Melan-A, and p63. Which of the following is the most definitive diagnosis established by these histopathologic and immunohistochemical findings?