**Core Concept:** Ankylosing spondylitis is a type of inflammatory arthritis that primarily affects the spine, sacroiliac joints, and other axial joints. It is a part of the group of diseases known as the Spondyloarthritis (SpA) group.

**Why the Correct Answer is Right:** Ankylosing spondylitis primarily affects the sacroiliac joints (SIJ) and the spine, causing inflammation and fusion of these joints over time. This results in pain, stiffness, and limited mobility in the lower back and pelvis region.

The least common joint to be affected in ankylosing spondylitis is the **knee joint**. Although knee involvement can occur, it is significantly less common compared to the other joints mentioned above. The knee joint is not considered a primary target in ankylosing spondylitis.

**Why Each Wrong Option is Incorrect:**

A. The **ankle joint** is not the least common joint to be affected in ankylosing spondylitis. While ankle joint involvement can occur, it is still more common compared to the knee joint.

B. The **shoulder joint** is also not the least common joint affected in ankylosing spondylitis. Although shoulder joint involvement is relatively rare, it is still more likely than knee involvement.

C. The **elbow joint** is less likely to be affected in ankylosing spondylitis compared to the other joints mentioned, but it is still more common than the knee joint.

D. The **hip joint** is less common than the ankle, shoulder, and elbow joints in ankylosing spondylitis; however, it is still more likely to be involved than the knee joint.

**Clinical Pearl:** Ankylosing spondylitis primarily affects the axial skeleton, including the spine, sacroiliac joints, and other spine segments. While it can involve peripheral joints like the ankle, knee, elbow, shoulder, and hip joints, the knee joint is the least commonly involved compared to these other peripheral joints.

**Correct Answer:** D. Hip joint.

**Explanation:** While the hip joint is less commonly involved than the ankle, knee, elbow, and shoulder joints in ankylosing spondylitis, it is still more likely to be affected than the knee joint. The hip joint is primarily involved in more severe cases of the disease, while the knee joint is more commonly involved in milder cases. This difference in severity is likely due to the knee joint's increased mobility compared to the hip joint, which has fewer degrees of freedom. Additionally, the hip joint is more protected by surrounding muscles and ligaments.