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: Surgery
Gold standard for diagnosis of osteoporosis ? 
Select and explain. 
A. DEXA, B. Single beam densitometry, C. Quatitative computed tomography, D. Bone histomorphometry
A: DEXA
Answer: A: DEXA 
Explanation: Ans. is 'a' i.e. The gold standard for diagnosis of osteoporosis is dual energy X-ray absorption scan (DEXA scan). The test is performed by passing low energy X-rays through a bone (e.g. spine, hip or wrist). The test takes about ten minutes, is painless, and is associated with very limited radiation exposure. The values generated by the test can then be compared to both: Young adult population : called a "T score", this test measures the variance between the patient and the young adult baseline. A score above -1 is considered normal; a score between -1 and -2.5 is considered osteopenia; and a score below -2.5 is considered osteoporosis. For each -1 standard detion in T score there is a 3 times increased risk of hip fracture and a 2.5 times risk of spine fracture. Age-and gender-matched control groups : a "Z score" measures the variance between the patient's and control groups' amount of bone. The control group consists of other people in the patient's age group of the same size and gender. An unusually high or low score may indicate the need for additional tests.