Longitudinal melanonychias.

Melanonychia is black or brown pigmentation that appears in the fingernails and toenails. The pigment can come from exogenous sources, such as bacteria or fungal infection, tar, or blood. Endogenous causes include aberrant melanin production in the nail bed, resulting in a longitudinal presentation. Melanonychia can indicate the presence of cancerous growths, as well as infection. Diagnostic measures, including dermatoscopy, biopsy, and histopathology, can determine the cause and direct the course of treatment. Malignant lesions should be excised, and underlying infections should be addressed with antibiotics or antifungals. Benign lesions and hyperpigmentation may benefit from a wait-and-see approach.