What is the most likely diagnosis for a 26-year-old man with knee pain and a sclerotic lesion on X-ray?

The most likely diagnosis for a 26-year-old man with knee pain and a sclerotic lesion on X-ray is osteosarcoma. Osteosarcoma is a malignant bone tumor that typically occurs in adolescents and young adults. It is commonly located around the knee joint, especially in the distal femur or proximal tibia.

The presentation of knee pain, coupled with the finding of a sclerotic lesion on X-ray, aligns well with osteosarcoma, as these tumors often exhibit a mixture of lytic and sclerotic bone changes. In this age group, osteosarcoma is the most prevalent primary malignant bone tumor, which further supports this diagnosis.

Other conditions mentioned may present with bone lesions, but they have different characteristics and occurrences. For example, chondrosarcoma typically occurs in older adults and is more often seen in the pelvis or proximal long bones, not primarily around the knee in a young adult. Osteochondroma is a benign bone growth that can appear in the knee area, but it usually manifests as a cartilage-capped bony projection rather than a sclerotic lesion. Synovial sarcoma, while possible in the knee region, is a soft tissue tumor that may not present with