Rhabdomyomatous mesenchymal hamartoma developed at an unexpected location.

Rhabdomyomatous mesenchymal hamartoma is a rare dermal lesion which was first described in 1986 as "striated muscle hamartoma". It usually develops in the head and neck region of newborns. We report a 38-day-old girl with a congenital skin tag in the perianal region. Physical examination did not reveal any congenital abnormalities or other dermal lesions. Histopathological examination showed a hamartoma with disorganized skeletal muscle fibers. The differential diagnosis includes skin tag, accessory tragus and soft fibroma. Rhabdomyomatous mesenchymal hamartoma differs from the listed lesions with its striated muscle component. Since it does not carry the risk of recurrence and malignant transformation, it is not very important to distinguish it from these lesions. However, a correct diagnosis is important because approximately one third of the cases are associated with congenital anomalies. Also, histopathological diagnosis should be made in children with perianal localization due to similar clinical manifestation of rhabdomyosarcoma.