Dark lines under fingernails can have various causes, just like most other symptoms. Finding the true cause means ruling out or confirming each possibility – in other words, diagnosis.
Diagnosis is usually a complex process due to the sheer number of possible causes and related symptoms. In order to diagnose dark lines under fingernails, we could:
Do you have any dark lines/streaks below the nails, running in the direction of nail growth?
Possible responses:→ No / don't know