A Long Brown Line Under A Young Woman's Fingernail

What is in your differential diagnosis for a thin brown line of parallel bands, regular in width that span nail fold to free nail edge?

The vertical brown line under the index fingernail of a African-American teen girl has not been cause for alarm over the 4-5 years she has been a patient at this clinic. The teen's sports participation physical examination with a resident in the room offers a teaching opportunity this physician can't pass up. What does this line look like to you? Is it ominous or otherwise benign? What's in your differential -- and what's your diagnosis?