Proximal white subungual onychomycosis: The rarest form of fungus infection of the finger or toenail. (Fungus infection of the finger or toenail is also called onychomycosis.) The infection begins in the nail fold (the portion of the nail opposite the tip of the finger).
The most common fungus causing the infection is called Trichophyton rubrum. Other causes include T. megninii, T. tonsiurans, T. mentagrophytes, T. schoenleinii, and Epidermophyton floccosum.
Proximal white subungual onychomycosis is diagnosed based upon its unusual location (in the portion of the nailbed closest to the hand). The diagnosis is confirmed by seeing the fungus under a microscope from a scraping of the tissue.