**Core Concept**
Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton, characterized by the formation of syndesmophytes, which are bony growths that bridge the intervertebral discs, leading to spinal fusion and reduced mobility.

**Why the Correct Answer is Right**
The presence of raised ESR and marked stiffness in the patient, combined with the finding of syndesmophytes on x-ray, are characteristic features of ankylosing spondylitis. The disease typically affects young adults but can also occur in older individuals. The formation of syndesmophytes is a hallmark of AS, leading to the fusion of vertebrae and resulting in reduced chest movements.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** Other conditions like osteoporosis or degenerative disc disease may cause back pain and stiffness, but they typically do not result in the formation of syndesmophytes.
* **Option C:** Rheumatoid arthritis (RA) is an autoimmune disease that primarily affects the peripheral joints, not the axial skeleton, and does not typically cause the formation of syndesmophytes.

**Clinical Pearl / High-Yield Fact**
The presence of syndesmophytes on x-ray is a key diagnostic feature of ankylosing spondylitis, and their presence should prompt further evaluation for this condition, especially in patients with chronic back pain and stiffness.

**Correct Answer:** C.