A 29-year old male patient presents to your clinic after 3 months of medial left knee pain that is worse when standing for a long period of time and after his weekly workgroup kickball games. Initially, he thought it was arthritis in his knee, but there is associated swelling on the inside of his knee that made him concerned. He denies any neurologic symptoms or unexplained weight loss. Figures A are X-rays and select MRI cuts performed in your clinic. Figure B is a biopsy specimen. What is the tissue of origin for this lesion?