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': "Osteoporosis is characterized by - \nSelect and explain. \nA. Increased serum alkaline phosphatase, B. Decreased bone density, C. Wasting of muscles, D. Looser's zone seen", 'role': 'user'}, {'content': 'Answer: B \nExplanation: Osteoporosis is characterized by an abnormally low bone mass (low bone density) and defects in bone structure, a combination of which renders the bone unusually fragile and at greater than normal risk of fracture.', 'role': 'assistant'}]