**Core Concept**
The intervertebral disc is a crucial component of the spine, consisting of two main parts: the annulus fibrosus and the nucleus pulposus. The annulus fibrosus is a tough outer layer, while the nucleus pulposus is a soft, gel-like center that provides shock absorption and flexibility to the spine.

**Why the Correct Answer is Right**
The nucleus pulposus is a highly compressible, gel-like substance within the intervertebral disc. When the disc is subjected to excessive stress or strain, the nucleus pulposus can herniate or bulge through the annulus fibrosus, compressing adjacent nerve elements. This compression can lead to severe back pain and neurological deficits, as seen in this patient. The intervertebral foramen, a narrow opening between adjacent vertebrae, allows the exiting nerve roots to pass through, making it a common site for nerve compression due to disc herniation.

**Why Each Wrong Option is Incorrect**
**Option A:** The annulus fibrosus is a tough, outer layer of the intervertebral disc, which is less likely to herniate or cause nerve compression.

**Option C:** The posterior longitudinal ligament is a fibrous structure that runs along the posterior aspect of the vertebral bodies, providing stability to the spine. While it can be a site of spinal stenosis, it is not typically responsible for compressing nerve elements at the L5-S1 level.

**Option D:** The anterior longitudinal ligament is a strong, fibrous structure that runs along the anterior aspect of the vertebral bodies, providing stability to the spine. It is not typically involved in compressing nerve elements at the L5-S1 level.

**Clinical Pearl / High-Yield Fact**
In patients with suspected lumbar disc herniation, a thorough physical examination, including a straight leg raise test, can help identify nerve root compression. This test involves lifting the patient's leg to assess for sciatica or radiculopathy.

**✓ Correct Answer: B. Nucleus pulposus**