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A 12-day-old Male With a "Skin Tag" on the Chin: Answer.

DISCUSSION Hair follicle nevus is a rare, benign, congenital lesion initially described by Pippione et al in 1984.1 Some authors have referred to this lesion as a congenital vellus hamartoma.2 Although this entity is relatively well described in the clinical dermatology literature (approximately 20 case reports), it is not well recognized in the dermatopathology community.3 Hair follicle nevus is a solitary hamartoma that is typically found on the head and neck region of infants. There is a debate on the exact classification of the lesion, as some authors believe that accessory tragus and hair follicle nevus are on a spectrum.3 The main differential diagnosis of a hair follicle nevus includes trichofolliculloma versus an accessory tragus. In general, multiple step sections and careful evaluation of histological sections are required to make the distinction between these lesions. Classically, trichofolliculloma histologically emanates from a central cyst with numerous smaller vellus hairs radiating outward. The clinical presentation differs as trichofolliculloma present as a hair containing lesion in adults. Accessory tragus and hair follicle nevus are both congenital lesions that often occur in the same location. Accessory tragi are predominantly located in the preauricular area (lesser extent the mandibular cheek and anterolateral neck), whereas the hair follicle nevus involves the head and neck location, following the distribution of the first branchial arch inferiorly to the chin. Microscopically, the presence of a cartilaginous core and stromal fat are diagnostic for an accessory tragus. Hair follicle nevi histologically contain normal-appearing mixed telogen and or vellus hairs. Sebaceous glands are usually present with rare eccrine glands.3 Although there is overlap with these 2 lesions, a cartilaginous component or central cyst has never been documented in a case of hair follicle nevus.3 The histological spectrum of findings among the lesions highlights the importance of careful evaluation of multiple step sections through the block. In conclusion, hair follicle nevi are rare, benign, hamartomatous lesions that may be clinically and histologically underdiagnosed. We would like to highlight the histological features and approach to sign-out in hopes of increasing awareness within the dermatopathology community.