**Core Concept**
Lumbar disc prolapse is a condition where the intervertebral disc between the lumbar vertebrae herniates, putting pressure on the surrounding nerves, spinal cord, or nerve roots. Accurate diagnosis of the extent and location of the prolapse is crucial for effective management.

**Why the Correct Answer is Right**
Magnetic Resonance Imaging (MRI) is the investigation of choice for lumbar disc prolapse due to its ability to provide detailed images of soft tissues, including the intervertebral discs, spinal cord, and surrounding nerves. MRI scans can accurately identify the extent of disc herniation, nerve root compression, and any associated spinal stenosis or spondylolisthesis. The high-resolution images obtained with MRI enable clinicians to plan appropriate treatment, including surgery, physical therapy, or conservative management.

**Why Each Wrong Option is Incorrect**
**Option A:** X-ray is not the best choice for diagnosing lumbar disc prolapse as it primarily shows bone structures and does not provide detailed images of soft tissues, such as intervertebral discs or nerves.

**Option B:** CT Scan can show disc herniation, but it is not as sensitive as MRI for detecting soft tissue abnormalities and may not provide adequate detail for planning surgical interventions.

**Option D:** Myelogram is an invasive procedure where a contrast agent is injected into the spinal canal to visualize the nerve roots and spinal cord. While it can provide some information on nerve root compression, it is less accurate than MRI and carries a higher risk of complications.

**Clinical Pearl / High-Yield Fact**
In patients with lumbar disc prolapse, MRI is also useful for detecting associated conditions such as spinal stenosis, spondylolisthesis, or facet joint arthritis, which may require separate management strategies.

**✓ Correct Answer: C. MRI**