A 25-year-old man has had right knee pain with "popping and catching" for the past 2 years. On examination, there is a reduced range of motion at the right knee, but no tenderness or swelling. MR imaging reveals a 3-cm mass in the anterior joint space. Arthroscopic surgery with partial synovectomy is performed, and a nodular, encapsulated mass is removed. Microscopic examination of the mass shows synoviocyte-like tumor cells in a hyalinized stroma containing osteoclast-like giant cells and hemosiderin. An antagonist to which of the following growth factors is most useful in treating this lesion?