A 14-year-old girl is brought to dermatology by her mother, following referral from her pediatrician for evaluation of fingernail changes. Initially, a faint brown streak was observed in the nail of her left fourth finger. Over time, the spot darkened and widened, and the adjacent nail plate flattened.
A few weeks before presentation to dermatology, the adjacent cuticle darkened. The patient denies any sensitivity in the area. Her mother denies any significant or relevant medical history.
EXAMINATION
The dark brown streak measures 4.5 mm in width and runs longitudinally the entire length of the nail plate. The discolored area coincides exactly with flattening of the nail plate, which, when viewed on edge, also exhibits darkening. Significantly, the adjacent cuticle is similarly affected.
No changes are observed on other digits. The child is of Native American ancestry, with type IV skin.
What is the diagnosis?