**Core Concept**
Osteoporosis is a systemic skeletal disorder characterized by a decrease in bone mass and density, leading to an increased risk of fractures. The gold standard for diagnosing osteoporosis involves measuring bone mineral density (BMD) to assess the risk of fractures.

**Why the Correct Answer is Right**
The gold standard for diagnosing osteoporosis is a dual-energy X-ray absorptiometry (DXA) scan, which measures BMD at the lumbar spine and hip. This test is considered the most accurate method for assessing bone density and predicting the risk of fractures. The DXA scan uses low-energy X-rays to measure the attenuation of bone and soft tissue, providing a BMD T-score that is compared to a young adult reference population. A T-score of -2.5 or lower is generally considered diagnostic of osteoporosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Bone biopsy is not the gold standard for diagnosing osteoporosis, as it is an invasive procedure that is not practical for routine clinical use. While bone biopsy can provide information on bone turnover and histomorphometry, it is not a widely accepted method for diagnosing osteoporosis.
**Option B:** Plain radiographs, such as X-rays, are not sensitive enough to detect early changes in bone density and are not considered a reliable method for diagnosing osteoporosis. They may show signs of osteoporosis, such as vertebral compression fractures, but are not a diagnostic gold standard.
**Option C:** Quantitative computed tomography (QCT) scans can measure bone density, but they are not as widely available or cost-effective as DXA scans, making them a less practical choice for routine diagnosis.

**Clinical Pearl / High-Yield Fact**
When interpreting BMD results, it's essential to consider the patient's clinical history, risk factors, and other diagnostic tests to determine the presence and severity of osteoporosis.

**Correct Answer: D. Dual-energy X-ray absorptiometry (DXA) scan.**