**Core Concept**
The patient's presentation of chronic backache and D12 collapse on X-ray, with maintained intervertebral disk space, suggests a vertebral compression fracture (VCF). VCFs can occur due to osteoporosis, trauma, or other underlying conditions. Maintained intervertebral disk space indicates that the fracture is likely a result of a compression force rather than a distractive force.

**Why the Correct Answer is Right**
The correct answer is **Vertebral Osteomyelitis**. Vertebral osteomyelitis is an inflammatory condition of the vertebral body, typically caused by bacterial infection (e.g., Staphylococcus aureus). It often presents with back pain, fever, and elevated inflammatory markers. However, it usually leads to destruction of the intervertebral disk space due to the inflammatory process.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Schmorl's node** is a herniation of the nucleus pulposus through the endplate of the adjacent vertebral body, often seen in degenerative disc disease. It can cause a compression fracture-like appearance on X-ray but typically maintains the intervertebral disk space.
* **Option B:** **Osteoblastoma** is a rare, benign bone tumor that can cause vertebral body collapse. However, it might not necessarily lead to destruction of the intervertebral disk space.
* **Option C:** **Ewing's sarcoma** is a malignant bone tumor that can cause vertebral body collapse. Like osteoblastoma, it might not necessarily lead to destruction of the intervertebral disk space.

**Clinical Pearl / High-Yield Fact**
In patients with vertebral compression fractures, it's essential to consider underlying conditions such as osteoporosis, trauma, or malignancy. Maintaining a high index of suspicion for these conditions will aid in prompt diagnosis and management.

**Correct Answer: C. Ewing's sarcoma**