Brief answer....it is called as melanonychia. Detailed answer... Hello, Thanks for the query. Melanonychia is actually the presence of melanin pigment in the nail plate. It generally presents as longitudinal band but can cover the entire nail or may present as transverse band also. Sometimes melanonychia can occur due to trauma,fungal infections,drugs like anti malarials and sometimes due to skin conditions like psoriasis or lichen planus. For proper diagnosis,you would have to consult a dermatologist ,who can perform a biopsy and get to the diagnosis. I hope it helps however you may revert to me for any further query. Best of luck Thanks

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