**Core Concept**
The vertebral canal is a cylindrical structure formed by the posterior elements of the vertebrae. It is divided into three parts: anterior (anterior two-thirds), middle, and posterior (posterior one-third). The posterior part of the vertebral canal is formed by different vertebrae and ligaments.

**Why the Correct Answer is Right**
The posterior one-third of the vertebral canal is formed by the posterior elements of the lumbar vertebrae (L1-L3) and the sacrum (S1-S5). This part of the canal is wider and more elliptical in shape compared to the anterior two-thirds. The posterior elements of the vertebrae, including the pedicles, laminae, and spinous processes, contribute to the formation of the posterior one-third of the vertebral canal. The ligamentum flavum, a strong fibroelastic ligament, connects the laminae of adjacent vertebrae and helps to form the posterior part of the canal.

**Why Each Wrong Option is Incorrect**
* **Option A:** The anterior two-thirds of the vertebral canal are formed by the vertebral bodies and intervertebral discs. This option is incorrect because it refers to the anterior part of the canal.
* **Option B:** The middle part of the vertebral canal is formed by the posterior elements of the thoracic vertebrae (T6-T12). This option is incorrect because it refers to the middle part of the canal, not the posterior one-third.
* **Option C:** The posterior one-third of the vertebral canal is formed by the posterior elements of the cervical vertebrae (C4-C7). This option is incorrect because the posterior one-third of the canal is formed by the lumbar vertebrae and sacrum, not the cervical vertebrae.

**Clinical Pearl / High-Yield Fact**
The vertebral canal is a critical structure that houses the spinal cord and nerve roots. It is essential to maintain the patency of the vertebral canal to prevent nerve compression and spinal cord injury. The posterior one-third of the vertebral canal is a common site for spinal stenosis and nerve root compression.

**Correct Answer:** D. (Note: The correct answer option is missing in the original question. Assuming it is D, the explanation is provided accordingly.)