This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous, Subject name: Orthopaedics
What is the World Health Organization (WHO) definition of osteoporosis? 
Select and explain. 
A. Bone mineral density less than 1 standard deviation below the mean of a young, healthy adult., B. Bone mineral density at least 2.5 standard deviations below the mean of a young, healthy adult., C. T score less than -2.5., D. (B) and (C).
D: (B) and (C).
Answer: D: (B) and (C). 
Explanation: Ans. D. (B) and (C).In 1994, the WHO introduced definitions for osteoporosis and osteopenia in terms of T score thresholds of -2.5 and -1 respectively. A T score of -2.5 means that the bone mineral density in the lumbar spine (L2-L4) is at least 2.5 standard deviations below the mean of a young (age 25-35), healthy female. A T score of more than -2.5 implies it is more positive and hence could represent either osteopenia or normal bone density.