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': 'Bone density is decreased in which of the following - \nSelect and explain. \nA. Osteoporosis, B. AVN of bone, C. 0steopetrosis, D. Fracture and collapse of cancellous bone', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Osteoporosis is characterized by an abnormally low bone mass (reduced bone density) and defects in bone structure.', 'role': 'assistant'}]