## **Core Concept**
Ankylosing spondylitis (AS) is a type of seronegative spondyloarthritis characterized by inflammation of the spine and sacroiliac joints, leading to eventual fusion of vertebrae. The condition primarily affects the axial skeleton, although it can also involve peripheral joints and extra-articular structures. The hallmark of AS is the involvement of the sacroiliac joints and the spine.

## **Why the Correct Answer is Right**
The correct answer, ., refers to the joints of the upper limb. In ankylosing spondylitis, the disease process predominantly affects the lower back and sacroiliac joints. While peripheral joints can be involved, especially in patients with peripheral arthritis, the joints of the upper limb are not typically a primary site of involvement in AS.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . The sacroiliac joints are characteristically involved in ankylosing spondylitis, making this option incorrect as it is a hallmark of the disease.
- **Option B:** . The spine is frequently involved in AS, with inflammation leading to stiffness and potential fusion of vertebrae, so this option is incorrect.
- **Option C:** . The hip joint can be involved in ankylosing spondylitis, particularly in more severe cases, leading to significant disability. Therefore, this option is incorrect.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl in ankylosing spondylitis is the association with HLA-B27, although it's worth noting that not all patients with AS are HLA-B27 positive, and not all HLA-B27 positive individuals develop AS. A classic presentation is a young male presenting with chronic back pain and stiffness, especially worse in the morning.

## **Correct Answer:** . The joints of the upper limb.