Ive noticed a light black/brown thin line right in the middle of my left thumb, and theres no pain or itching. I dont remember injuring it. I saw a doctor (not a dermatologist), and he said it might be a fungal infection, so I took medicine for a week. The line didnt go away, but it didnt get darker either. When I went back, the doctor said its a bit lighter and told me not to worry about it. Its only on this thumb and not on any other fingers or my legs. Should I be concerned, or is it okay to just leave it? Do you think I should get a second opinion, especially from a dermatologist, or is it nothing urgent?

That could be due stress or pressure induced pigmentation,no need to worry unless it does progress otherwise u can ignore it.