¿Qué son esas finas líneas negras bajo la uña?

I’ve recently come across a client who has the smallest splinter looking like a black tiny thin line under the nail. Haven’t come across this before. What would/could this be?

We often see lines like that under our clients’ nails. They are called splinter hemorrhages and are essentially small spots of blood caused due to bleeding under the nails.

There is no physical pain or discomfort associated with the hemorrhages unless it has been caused by physical trauma like a bang to the uña natural. Usually, they will grow out along with the nail and within three or four months the hemorragia lines are no longer seen.