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What causes a black line in the finger nail?

My teenage daughter has a black line running from the cuticle of her thumbnail to the tip. She doesn't know how she got it, and I haven't seen anything like it. What is this phenomenon? Is it dangerous?
Lines and other deformities or color changes are relatively common, and most of them are not serious. This is particularly true if the line runs vertically, as in the case of your daughter. Horizontal lines running through the nail, on the other hand can sometimes indicate an underlying medical problem. Vertical ridges or lines in the nail often do not have a clear cause. Sometimes they may be genetic and require no treatment. These lines are usually clear or white, but can become dark in color if they have dirt or fingernail polish lodged in them. Occasionally, fungal infections of the nail will cause similar lines, although typically there is also more generalized deformity or thickening of the nail as well. Another possibility would be a foreign body under the fingernail, which can look like a black line. Common types of foreign bodies that can do this include wood splinters or pencil leads. Finally, rarely, bleeding can show up under the nails as small dark lines called splinter hemorrhages. This would most likely occur in multiple nails, however, but if there are any other signs of bleeding or bruising, fever, or other concerning signs this should be checked out. If you any doubts or concerns, your daughter's primary care doctor would be most likely happy to examine her nails.
This answer is for general informational purposes only and is not a substitute for professional medical advice.
If you think you may have a medical emergency, call your doctor or (in the United States) 911 immediately. Always seek the advice of your doctor before starting or changing treatment. Medical professionals who provide responses to health-related questions are intended third party beneficiaries with certain rights under ZocDoc’s Terms of Service.

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