**Core Concept**
Ankylosing spondylitis (AS) is a form of seronegative spondyloarthritis characterized by inflammation of the spine and sacroiliac joints, leading to progressive fusion of the vertebrae and eventual loss of spinal mobility. The disease often presents with morning stiffness, back pain, and improvement with exercise or warm baths.

**Why the Correct Answer is Right**
Decreased chest wall expansion is a common finding in patients with ankylosing spondylitis due to the fusion of the costovertebral joints, leading to a reduction in thoracic cavity size and impaired lung expansion. This is a result of the inflammatory process causing new bone formation (osteophytes) and eventual fusion of the spine, which restricts chest wall movement. As the disease progresses, patients may experience a decrease in lung capacity and vital capacity.

**Why Each Wrong Option is Incorrect**
**Option A:** Marrow fibrosis is not a characteristic finding of ankylosing spondylitis. Marrow fibrosis is more commonly associated with conditions such as myelofibrosis or bone marrow metastasis.
**Option B:** Distal phalangeal joint involvement is more typical of psoriatic arthritis, a different type of seronegative spondyloarthritis.
**Option C:** Pleural nodules can be associated with various conditions, including tuberculosis, rheumatoid arthritis, and systemic lupus erythematosus, but are not a characteristic finding in ankylosing spondylitis.

**Clinical Pearl / High-Yield Fact**
In patients with ankylosing spondylitis, the "bamboo spine" appearance on radiographs is a classic sign of the disease, indicating spinal fusion and calcification of the intervertebral discs.

**✓ Correct Answer: D. Decreased chest wall expansion**