**Core Concept**
Spinal tuberculosis (TB) is a rare but serious form of extrapulmonary TB that affects the spinal column. It is characterized by the destruction of vertebral bodies and the intervertebral discs, leading to spinal deformity and neurological deficits. The investigation of choice for diagnosing spinal TB involves imaging studies that can detect the characteristic changes in the spine.

**Why the Correct Answer is Right**
The investigation of choice for spinal TB is MRI (Magnetic Resonance Imaging) of the spine. MRI is highly sensitive and specific for detecting early changes in the spine, such as vertebral body destruction, disc space narrowing, and spinal cord compression. It can also help identify the extent of the disease and guide the treatment plan. In contrast, CT scans and plain X-rays are less sensitive for early detection and may not provide adequate information for treatment planning.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan of the spine. CT scans are useful for detecting bony destruction and calcification but are less sensitive than MRI for detecting soft tissue changes and early disease.
**Option B:** Plain X-ray of the spine. Plain X-rays are insensitive for detecting early changes in the spine and may not provide adequate information for treatment planning.
**Option C:** Bone scan. Bone scans are not specific for TB and may not provide adequate information for treatment planning.

**Clinical Pearl / High-Yield Fact**
MRI is the investigation of choice for spinal TB due to its high sensitivity and specificity for detecting early changes in the spine. It is essential to consider TB in the differential diagnosis of patients with chronic back pain, fever, and weight loss, particularly in regions with high TB prevalence.

**Correct Answer:** C. MRI of the spine.