Answer: Osteochondroma (exostosis)

Osteochondroma are common benign tumors resulting from growth plate cartilage displacement into the metaphyseal region. The underlying bone is normal and growth from the cartilage cap ceases with fusion of the epiphyses. Exostoses often have a stalk and a cauliflower-like head. Ninety percent are solitary lesions. Exostoses have a 1% risk for malignant transformation to chondrosarcoma. Growth after skeletal maturity suggests malignancy, which often presents with pain.