Acanthotic subtype of onychocytic matricoma - A rare case with unusual features

Onychocytic matricoma (OCM) is a benign tumor of the onychocyte. It clinically presents as localized longitudinal pachyonychia and melanonychia. It has many clinical and histopathological differential diagnoses. A 54-year-old male presented with a 2-year history of melanonychia on the left thumb. Local examination revealed a 2 mm skin colored papule over the proximal nail fold. The proximal nail plate showed longitudinal melanonychia with subungual hyperkeratosis. Histopathological examination of the nail bed revealed proliferation of basaloid epithelial cells. There were many organized spheres composed of prekeratogenous and keratogenous layers. No nuclear atypia was noted. The nail plate biopsy showed septate, thin-walled fungal hyphae, which were highlighted by periodic acid Schiff (PAS) stain. We report an additional case of the acanthotic subtype of onychocytic matricoma in skin of color. Additionally, this case also showed onychomycosis. We also discuss the literature review and diagnostic approach. Knowledge about this rare tumor will help to arrive at the correct diagnosis.