nevus comedonicus 

The nevus comedonicus  is an uncommon variant of adnexal hamartoma which appears clinically as linear groups of open comedones. Nevus comedonicus 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.