Answer: Blue nevus

Histology: The dermal lesion contains abundant melanin pigment, most of which is located within melanophages. There is a central area of thickened collagen. The characteristic finding is a population of dendritic-shaped melanocytes with fine pigment granules within their processes, dissecting collagen bundles of the reticular dermis. These are the typical features of a blue nevus.

Discussion: A cellular blue nevus is a biphasic lesion composed of clear cells in nests and fascicles along with the pigmented dendritic spindle cells that are seen in common blue nevus. Importantly, cellular blue nevi may have dermal mitoses, but this does not make them melanomas.

Malignant melanoma may have a morphology that mimics a blue nevus. In contrast to a blue nevus, true malignant melanomas show a lack of cytologic maturation as the cells descend into the dermis, have frequent mitoses, and have a “pushing” border.

A nevus of Ota is another of the dendritic melanocytoses, in other words a lesion composed of dendritic cells with melanin granules within their processes. However, the nevus of Ota occurs in a trigeminal nerve distribution, affecting facial skin and the ocular membrane.