Nevus comedonicus.

The nevus comedonicus (NC) is an uncommon variant of adnexal hamartoma which appears clinically as linear groups of open comedones. Its name may be a misnomer since, according to some, true comedones are not present. NC usually occurs by itself but may be linked with a variety of systemic findings such as skeletal or ocular anomalies. Although the nevus comedonicus is viewed by many as a hamartoma arising from a defective mesoderm, others consider this lesion to be an epidermal nevus involving the hair follicle or an appendageal nevus of sweat ducts.