Dark streaks can follow injury that bleeds under the nail. If no trauma occurred, see a doctor fast; an unexplained line may signal skin cancer. Called melanonychia, these stripes are worrisome when they appear on one nail. Some lines link to medication, pregnancy, or genetics, but any new or changing mark deserves prompt evaluation. Subungual melanoma is the gravest risk, so treat every widening or darkening band as urgent.
