Nevus sebaceus is a distinct clinicopathologic entity. It is often present at birth on the head and neck and then comes more warty with age. It is usually isolated, as in this case, but may be part of a syndrome. The diagnosis can often be made on histology alone when it shows all the features as in this case, but at times it may resemble an epidermal nevus or seborrheic keratosis. The clinical appearance of the lesion and age of the patient are helpful clues.
There is an epidermal lesion, showing mild acanthosis, papillomatosis, and orthokeratotic hyperkeratosis. There are primitive follicular structures in the superficial dermis, but no mature anagen follicles or emanating hairs. Sebaceous lobules are prevalent, some appearing to directly connect with the overlying epidermis. Apocrine galnds are also noted.