A rare benign nail tumor originating in the nail matrix characterized by localized pachyonychia and variable degrees of pigmentation: pigmented, melanocytic (common, longitudinal melanonychia that may simulate a foreign body) or hypopigmented. Histopathology demonstrates a purely epithelial tumor with endo-keratinization in the deep portion and concentrically arranged nests of pre-keratogenous and keratogenous cells. [from
SNOMEDCT_US]