Which of the following is the most common site of origin for osteosarcoma in the long bones?

Explanation

## Anatomical Origin of Osteosarcoma **Key Point:** Osteosarcoma arises in the **metaphyseal region** of long bones, most commonly around the knee joint (distal femur and proximal tibia account for ~50% of cases). ### Why the Metaphysis? The metaphysis is the site of most active bone growth and remodeling, particularly during adolescence and early adulthood when osteosarcoma incidence peaks. The high mitotic activity and rapid bone turnover in this region predispose to malignant transformation. ### Common Sites of Osteosarcoma | Site | Frequency | Notes | | --- | --- | --- | | Distal femur | ~40% | Most common single site | | Proximal tibia | ~15% | Around the knee (combined ~55%) | | Proximal humerus | ~10% | Second most common bone | | Pelvis | ~8% | Often presents late; worse prognosis | | Distal tibia | ~5% | Less common than proximal | **Clinical Pearl:** The "around the knee" region (distal femur + proximal tibia) accounts for approximately **half of all osteosarcomas**, making it the most important anatomical landmark to remember for exam purposes. **High-Yield:** Osteosarcoma occurs in areas of **rapid bone growth**—hence the peak incidence in adolescents during growth spurts and the predilection for the metaphyseal regions of long bones. [cite:Robbins 10e Ch 24]