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': "A 12 year old girl complains of pain persisting in his leg for several weeks with a low grade fever. A radiograph reveals a mass in the diaphyseal region of the left femur with overlying cortical erosion and soft tissue extension. A biopsy of the lesion shows numerous small round cells, rich in PAS positive diastase sensitive granules. The most likely histological diagnosis is - \nSelect and explain. \nA. Osteogenic sarcoma, B. Osteoblastoma, C. Ewing's sarcoma, D. Chondroblastoma", 'role': 'user'}, {'content': 'Answer: C \nExplanation: Small round cells rich in PAS positive diastase sensitive granules confirms the diagnosis of Ewings sarcoma.', 'role': 'assistant'}]