Answer: Epidermal nevus

Histology: Histologically, epidermal nevi show hyperkeratosis, papillomatosis and mild acanthosis all features of a simple squamous papilloma. They can also resemble a seborrheic wart or rarely show epidermolytic hyperkeratosis. No viral cytopathic changes or significant epithelial atypia is usually seen. Variable degree of inflammation can be present.

Discussion: Epidermal nevi can be subdivided clinically into three forms, all of which are identical histologically and only differ in the extent of involvement of the affected area. Lesions that are present at birth usually develop in childhood, and are yellow-brown warty papules or plaques with an irregular margin. They usually affect the trunk or limbs and may cause a cosmetic problem.