A 24-year-old woman has a 1.5 cm vulvar nodule that is dark blue and slightly raised. Histology shows nests of large cells with abundant eosinophilic cytoplasm and round nuclei within the dermis. Cells are positive for S100 and HMB-45. Which is the diagnosis?

• A Malignant melanoma
• B Metastatic melanoma
• C Cellular blue nevus
• D Vulvar blue nevus ✓

Explanation

Vulvar blue nevus presents as a small, dark blue, slightly raised vulvar nodule, typically in young women. Histology shows bland dendritic melanocytes in the dermis, positive for S100 and HMB-45, without atypia or mitoses. Malignant melanoma shows cytologic atypia, mitoses, and often ulceration. Metastatic melanoma would have a known primary. Cellular blue nevus is larger, more cellular, and usually occurs in other sites.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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