**Core Concept**
Ankylosing spondylitis (AS) is a chronic inflammatory disease that primarily affects the axial skeleton, leading to fusion of the spine and sacroiliac joints. It is characterized by a combination of inflammatory, degenerative, and fibrotic changes in the affected joints.

**Why the Correct Answer is Right**
The correct answer is not a feature of AS because it does not accurately describe a radiological manifestation of the disease. AS is characterized by several distinctive radiological features, including:
* Sacroiliitis: inflammation of the sacroiliac joints, leading to erosion and sclerosis of the joint margins.
* Spinal involvement: fusion of the vertebral bodies, leading to bamboo spine appearance on radiographs.
* Syndesmophytes: calcified, bony growths that form along the ligaments and tendons of the spine.
* Squaring of the vertebrae: due to the formation of new bone along the edges of the vertebral bodies.

**Why Each Wrong Option is Incorrect**
**Option A:** This feature is a hallmark of AS, characterized by inflammation and eventual fusion of the sacroiliac joints.
**Option B:** This feature is a classic radiological finding in AS, where the vertebral bodies become increasingly fused, leading to a bamboo spine appearance.
**Option C:** This feature is not typically associated with AS, which is characterized by inflammation and fusion of the joints, rather than degenerative changes.
**Option D:** This feature is not a characteristic of AS, which is primarily an inflammatory disease, rather than a degenerative one.

**Clinical Pearl / High-Yield Fact**
A key radiological feature of AS is the "bamboo spine," which is characterized by the fusion of the vertebral bodies, giving the spine a characteristic appearance on radiographs.

**Correct Answer:** C