**Core Concept**
The question is testing the understanding of syndesmophytes and their association with a specific spinal condition. Syndesmophytes are bony growths that bridge the disc space and can be seen in conditions like ankylosing spondylitis, where there is inflammation and new bone formation in the spine.

**Why the Correct Answer is Right**
The presence of syndesmophytes involving 4 continuous vertebrae on X-ray suggests a diagnosis of ankylosing spondylitis (AS). AS is a chronic inflammatory disease that primarily affects the spine, leading to new bone formation and eventual fusion of the vertebrae. The continuous involvement of 4 vertebrae is characteristic of AS, as opposed to other conditions like degenerative disc disease, which typically affects individual discs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the question, as it does not provide a condition that would explain the presence of syndesmophytes.
**Option B:** Osteoporosis is a condition characterized by bone thinning and fragility, but it would not typically present with syndesmophytes or continuous vertebral involvement.
**Option C:** Degenerative disc disease can cause back pain and disc space narrowing, but it does not typically involve the formation of syndesmophytes or continuous vertebral involvement.

**Clinical Pearl / High-Yield Fact**
In AS, the presence of syndesmophytes and continuous vertebral involvement can be a sign of impending spinal fusion and potential loss of spinal mobility.

**Correct Answer:** B. Ankylosing spondylitis.