Hello, I'm 17 and over the past few months I've had a faint black nail on my right thumb nail. I went to get it checked and she told me I have a vitamin deficiency. It's not in the root of my skin it's just on my nail. But everytime my nail grows the line grows with it. Since yesterday under my nail has been sore and my thumb has been too I was just wondering if it's serious I used to bite my nails a lot too.
