**Core Concept:** Ankylosing spondylitis (AS) is a chronic inflammatory autoimmune disease that primarily affects the axial skeleton, particularly the spine and sacroiliac joints. It is a form of spondyloarthritis, which is a group of conditions characterized by inflammation of the spine and other joints.

**Why the Correct Answer is Right:** Ankylosing spondylitis is primarily associated with the HLA-B27 antigen. This is a specific human leukocyte antigen (HLA) gene found on chromosome 6, which is part of the Major Histocompatibility Complex (MHC) class I. The presence of HLA-B27 increases the risk of developing AS, but it is not a definitive diagnosis.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** Spondylitis (inflammation of the spine) is a symptom and not a specific association with AS.
B. **Option B:** Spondylitis is not a specific association with HLA-B27 and AS.
C. **Option C:** Ankylosing spondylitis is related to HLA-B27, not human leukocyte antigen (HLA) itself.
D. **Option D:** While ankylosing spondylitis is associated with HLA-B27, it is not solely responsible for the disease. Other factors contribute to the development of AS.

**Clinical Pearl:** Understanding the association between HLA-B27 and ankylosing spondylitis is crucial for making an accurate diagnosis and providing appropriate management strategies for patients presenting with AS symptoms.

**Correct Answer:** D. Ankylosing spondylitis is associated with HLA-B27, but it is not solely responsible for the disease. Other factors contribute to the development of AS.