This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Orthopaedics
[{'content': 'Babu a 19 yrs old male has a small circumscribed sclerotic swelling over diaphysis of femur; likely diagnosis is - \nSelect and explain. \nA. Osteoclastoma, B. Osteosarcoma, C. Ewing sarcoma, D. Osteoid osteoma', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Well circumscribed sclerotic swelling over diaphysis of a long bone (femur) in a 19 years old suggests the diagnosis of osteoid osteoma.', 'role': 'assistant'}]