Longitudinal melanonychia caused by trichophyton rubrum. Histochemical and ultrastructural study of two cases.

Two patients had black pigmentation affecting a great toenail that clinically simulated longitudinal melanonychia; Trichophyton rubrum with a diffusible black pigment was consistently isolated from the nail lesions. Light microscopic examination of nail samples stained with hematoxylin-eosin-safranine showed hyphae containing a brown cytoplasmic pigment. The positive Masson-Fontana stain suggested that the pigment was related to melanin. Both histologic and ultrastructural studies demonstrated intracytoplasmic polymerization, a method of final enzymatic polymerization that is unusual for melanin-producing fungal strains.