**Core Concept**
Syndesmophytes are bony growths that occur in the spine, specifically in the ligaments and tendons that connect the vertebrae. They are a hallmark of a specific inflammatory condition affecting the spine.

**Why the Correct Answer is Right**
Syndesmophytes are characteristic of ankylosing spondylitis, a type of inflammatory arthritis that primarily affects the spine. In ankylosing spondylitis, the body's immune system mistakenly attacks the joints between the vertebrae, leading to inflammation and the formation of syndesmophytes. Over time, these growths can fuse the vertebrae together, causing stiffness and loss of mobility in the spine. The exact mechanism of syndesmophyte formation involves the activation of osteoblasts, which are bone-forming cells, in response to inflammation.

**Why Each Wrong Option is Incorrect**
**Option A:** Rheumatoid arthritis is an autoimmune disease that primarily affects the hands and feet, and is not typically associated with syndesmophytes.
**Option B:** Osteoarthritis is a degenerative joint disease that can affect any joint, but it does not lead to the formation of syndesmophytes.
**Option C:** Psoriatic arthritis is a type of inflammatory arthritis that can affect the spine, but it is not typically associated with syndesmophytes.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with back pain and stiffness, it's essential to consider the possibility of ankylosing spondylitis, especially if the patient has a family history of the condition or other associated symptoms such as uveitis or enthesitis.

**Correct Answer: D. Ankylosing spondylitis**