This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Tumors, Subject name: Orthopaedics
[{'content': "Most common cause of bone malignancy - \nSelect and explain. \nA. Secondaries, B. Osteosarcoma, C. Ewing's sarcoma, D. Osteoclastoma", 'role': 'user'}, {'content': 'Answer: A \nExplanation: OSTEOLYTIC SECONDARIES Tumor cells produce growtl factors that stimulate bone destruction i.e. RANK &;igen& Osteoclasts are activated and break down bone Osteoblasts cannot build bone back fast enough Decreased bone density and strength: high risk for fracture OSTEOBLASTIC SECONDARIES Osteoblasts are stimulated by tumors to lay down new bone Bone becomes abnormally dense and stiff Paradoxically bones are also at risk of breaking REF : MAHESWARI 9TH ED', 'role': 'assistant'}]