**Core Concept**
The calcification of the posterior longitudinal ligament (PLL) is a condition characterized by the deposition of calcium salts within the PLL, leading to spinal cord compression and potential neurological deficits. This condition is often associated with osteoporosis, spinal degeneration, or trauma.

**Why the Correct Answer is Right**
The calcification of the PLL is typically diagnosed using imaging modalities, with the most sensitive and specific method being **MRI**. MRI can provide detailed images of the soft tissues, including the PLL, and can detect the presence of calcification. The calcification appears as a high-signal intensity area on T1-weighted images and a low-signal intensity area on T2-weighted images.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray is not the best diagnostic tool for calcification of the PLL, as it may not be sensitive enough to detect the calcification, especially in early stages.
**Option B:** CT scan can detect calcification, but it is not as sensitive as MRI for soft tissue evaluation.
**Option C:** Plain radiographs may show some signs suggestive of calcification, but they are not specific or sensitive enough for a definitive diagnosis.

**Clinical Pearl / High-Yield Fact**
When evaluating patients with spinal cord compression, it is essential to consider the possibility of calcification of the PLL, especially in older adults with a history of osteoporosis or spinal degeneration.

**Correct Answer: C. CT scan. CT scan is often used to evaluate calcification of the posterior longitudinal ligament, although MRI is typically the preferred imaging modality.**