**Core Concept**
The underlying principle being tested is a common musculoskeletal disorder characterized by inflammation of the spinal joints (zygapophyseal joints), leading to pain and stiffness in the back.

**Why the Correct Answer is Right**
The patient's symptoms of back pain and delayed morning stiffness, along with the provided x-ray findings, are consistent with ankylosing spondylitis (AS). AS is a type of seronegative spondyloarthropathy (SpA) that primarily affects the axial skeleton, causing inflammation and eventual fusion of the spinal joints. The disease is associated with the presence of HLA-B27 antigen and can lead to significant spinal deformity and disability. The x-ray findings in AS typically show sacroiliitis, syndesmophytes, and eventual ankylosis of the spine.

**Why Each Wrong Option is Incorrect**
* **Option A:** Psoriatic spondylitis is another type of seronegative SpA, but it typically presents with skin psoriasis and dactylitis, which are not mentioned in the patient's history.
* **Option B:** Osteoarthritis (OA) is a degenerative joint disease that can cause back pain, but it usually affects older patients and is not associated with the same level of inflammation and morning stiffness seen in AS.
* **Option C:** Scoliosis is a structural deformity of the spine that can cause back pain, but it is not typically associated with the inflammatory changes and fusion seen in AS.

**Clinical Pearl / High-Yield Fact**
Ankylosing spondylitis often presents with a classic "bamboo spine" appearance on x-ray, where the spine becomes fused and resembles a bamboo stalk.

**Correct Answer:** C. Ankylosing spondylitis