Folliculosebaceous Cystic Hamartoma With Unusual Mucinosis: A Report of a Rare Case

We report a case of a clinically indistinct pink papule on the midline upper back of a 77-year-old woman. Histopathological examination of the lesion revealed a unique variant of folliculosebaceous cystic hamartoma (FSCH) characterized by mucin deposits in sebaceous lobules. FSCH is a rare lesion involving a cystically dilated follicle surrounded by sebaceous glands embedded within a stromal matrix composed of mesenchymal features. Follicular mucinosis is an uncommon inflammatory disorder typically presenting as benign, idiopathic lesions in children or mycosis fungoides-associated lesions in adults. A literature review revealed no previously reported cases of FSCH involving mucinosis of our knowledge to date, making this a novel and clinically relevant histopathologic variant. This case expands the histological presentation of a rare lesion and highlights the importance of careful examination in differentiating between similarly presenting dermatological conditions. Additionally, this novel finding suggests the possibility of an unknown pathologic mechanism of mucin deposition in FSCH, warranting further research.