**Core Concept**
Dual-energy X-ray absorptiometry (DEXA) scan is a non-invasive imaging technique used to measure bone mineral density (BMD), which is essential for diagnosing and managing osteoporosis. DEXA scans use low-energy X-rays to measure the density of bones, providing accurate information about bone health.

**Why the Correct Answer is Right**
A DEXA scan is specifically designed to measure bone density by detecting the attenuation of X-rays as they pass through the body. This technique is highly sensitive and accurate for assessing BMD, making it the gold standard for diagnosing osteoporosis. DEXA scans typically measure the BMD at the hip and spine, which are common sites for osteoporotic fractures. The results are expressed as a T-score, comparing the patient's BMD to that of a young adult of the same sex.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scans are not typically used for bone density measurements, as they involve higher doses of radiation and are more suited for visualizing bone structure and anatomy.

**Option C:** MRI scans are not the best choice for measuring bone density, as they are primarily used for imaging soft tissues and do not provide accurate measurements of bone mineral content.

**Option D:** Bone scans are used to detect bone metastases, infections, and fractures, but they are not suitable for measuring bone density. They involve injecting a radioactive tracer that accumulates in areas of high bone turnover.

**Clinical Pearl / High-Yield Fact**
When interpreting DEXA scan results, it's essential to remember that a T-score of -1 to -2.5 indicates osteopenia, while a T-score below -2.5 indicates osteoporosis.

**✓ Correct Answer: B. DEXA scan**