A pigmented nail presents a diagnostic challenge for the clinician because its differential diagnoses include minor and life-threatening disorders.1- 3 Normal nails are not pigmented because nail matrix melanocytes are quiescent and do not produce melanin. However, nail matrix melanocytes may be activated by a variety of different stimuli and start to produce melanin, which is incorporated into the nail plate and causes nail pigmentation. This is usually arranged in longitudinal streaks (longitudinal melanonychia), usually arising from the distal matrix; it is much more common in dark-skinned individuals than in whites. Common causes of longitudinal melanonychia due to melanocyte activation include inflammatory and traumatic nail disorders, drug reactions, and systemic diseases.
Tosti A, Argenziano G. Dermoscopy Allows Better Management of Nail Pigmentation. Arch Dermatol. 2002;138(10):1369-1370. doi:10.1001/archderm.138.10.1369