Hello, your disease here is called fingernail mole. In general, there is no problem. No need for treatment, just pay attention to observation. If the black line widens in a short time, you need to go to the hospital for treatment.

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There are many reasons for the formation of nail line/nail nevus. Systemic pigmented diseases and hyperpigmentation can be seen in acanthosis nigricans, Addison's disease, polyposis nigromaculata syndrome and vitamin B 12 deficiency after adrenalectomy for Cushing's syndrome or Logil-hunziker syndrome. The local inducing factors are trauma, fungal infection, eczema-like changes and hypothyroid bleeding. Other factors can be seen in the enhancement of melanocyte activity induced by drugs (chemotherapy drugs, antimalarials, minocycline or gold preparations), which can all lead to nail line, and should be diagnosed as nail line clinically.