Epithelioid histiocytic infiltrate caused by Mycobacterium scrofulaceum infection: a potential mimic of various neoplastic entities.

Mycobacterium scrofulaceum is a slow-growing atypical mycobacterium that is ubiquitous within our environment found in the water and soil. Most commonly, it manifests as the organism responsible for lymphadenitis in children. In adults, infection by this organism is rare and usually occurs in the setting of local or systemic host immunosuppression. We herein report a case of a 45-year-old woman who presented with a large subcutaneous nodule over her right upper arm. She had been on low dose oral prednisone for 17 years for systemic lupus erythematosus without complication. A biopsy of the nodule revealed a diffuse dermal infiltrate of epithelioid histiocytes laden with acid-fast bacilli mimicking a fibrohistiocytic neoplasm. Treatment with clarithromycin monotherapy resulted in clinical remission. Consideration of atypical mycobacterial infection is warranted in fibrohistiocytic proliferations of the skin especially within immunosuppressed patients.