**Core Concept**
Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton, particularly the sacroiliac joints and the spine. It is characterized by inflammation, pain, and eventual fusion of the vertebrae, leading to reduced spinal mobility and rigidity.

**Why the Correct Answer is Right**
The patient's symptoms, such as low back pain, stiffness, and limited flexion of the lumbar spine, are classic presentations of AS. The x-ray of the lumbar spine would likely show sacroiliitis and/or vertebral body squaring, which are hallmarks of the disease. AS often presents in young adults with a male preponderance, and symptoms may be exacerbated by night pain, hip pain, and paravertebral muscle tenderness. The disease is associated with HLA-B27, a genetic marker that increases the risk of developing AS.

**Why Each Wrong Option is Incorrect**

* **Option A:** Reiter syndrome, also known as reactive arthritis, is a type of spondyloarthritis that typically follows a bacterial infection. It presents with arthritis, urethritis, conjunctivitis, and skin lesions, but is not primarily associated with axial skeleton involvement or the characteristic radiographic changes seen in AS.
* **Option B:** Marfan syndrome is a genetic disorder affecting the body's connective tissue, leading to cardiovascular, ocular, and skeletal abnormalities. While it can affect the spine, Marfan syndrome is not associated with the inflammatory changes or sacroiliitis seen in AS.
* **Option D:** Rheumatoid arthritis (RA) is a systemic autoimmune disease primarily affecting the small joints of the hands and feet. While it can involve the spine, RA typically presents with symmetric joint involvement and is not characteristically associated with the axial skeleton or the specific radiographic features of AS.

**Clinical Pearl / High-Yield Fact**
The "Bamboo spine" on radiography is a classic sign of AS, characterized by the fusion of adjacent vertebrae, resulting in a "bamboo-like" appearance. This radiographic feature is a result of chronic inflammation and eventual bony fusion of the vertebral bodies.

**✓ Correct Answer:** C. ankylosing spondylitis (AS)