Mesenchymal–Follicular Hamartoma: A Case Report of a Cutaneous Hamartoma With Dominant Mesenchymal Overgrowth

Background: Cutaneous hamartomas are benign lesions composed of disorganized but mature tissue elements normally found at the site. Although many types are well defined, some rare variants do not conform to established categories. Case Presentation: We present a unique case of a cutaneous hamartoma with prominent mesenchymal overgrowth and minimal epithelial and adnexal components. Assessment: The mesenchymal component consisted of mature fibrocytes, adipocytes, and blood vessels. The epithelial/adnexal component was limited to a few terminal hairs with aberrant trichogenesis, reminiscent of trichofolliculoma, and small cystic apocrine ducts/glands resembling hidrocystoma. Diagnosis: We found it challenging to classify this lesion, because it did not conform to any well-known hamartomas. Given the prominent mesenchymal component and limited adnexal involvement, “mesenchymal–follicular hamartoma” may be the most concise and descriptive term, emphasizing the dominant mesenchymal component in our case. Conclusions: This case report highlights a rare presentation of a cutaneous hamartoma with dominant mesenchymal features. This predominance suggests that mesenchymal proliferation may play a primary role in the histogenesis of the lesion.