What Causes C Shaped Ring On The Inside Of Leg?

I have a C shape ring on the inside of my right leg. I saw a dermatologist last year and he had given me a strong cream to use. I was to use for one week, off two weeks I believe and then use one week again before I go back to see him. When I went back 4 weeks later the cream had done nothing. He said he could do a biopsy but I opted not. It doesn t hurt or itch it s just there. I feel now I would like to have a biopsy done to find out exactly what this is.