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': "Which of the following is a primary defect in Paget's disease? \nSelect and explain. \nA. Osteoblast, B. Osteoclast, C. Osteocyte, D. Fibroblast", 'role': 'user'}, {'content': "Answer: B \nExplanation: Paget's disease (Osteitis deformans)\n\nPaget's disease is characterized by increased bone turnover and enlargement and thickening of the bone, but the internal architecture is abnormal and the bone is usually brittle.\nPrimary defect is in osteoclasts with increased osteoclastic activity.\nThis results secondarily increase in osteoblastic activity (normal osteoclasts and osteoblasts act in a co-ordinated manner).\nSo, characteristic cellular change is a marked increase in osteoclastic and osteoblastic activity.\nBone turnover is accelerated, plasma alkaline phosphatase is raised (a sign of osteoblastic activity) and there is increased excretion of hydroxyproline in urine (due to osteoclastic activity).", 'role': 'assistant'}]