What is the primary tissue target in Ankylosing Spondylitis?

• A. Tendon
• B. Enthesis
• C. Cartilage
• D. Bone marrow

Answer: (B)

Enthesis—the insertion point where ligaments or tendons attach to bone—is the primary tissue target in Ankylosing Spondylitis. In AS, inflammation starts at these enthesial sites (enthesitis), especially around the spine and sacroiliac joints. This enthesitis explains the characteristic inflammatory back pain that worsens with rest and improves with activity, as well as heel pain from Achilles or plantar fascia insertions. Over time, chronic inflammation at the entheses promotes new bone formation, leading to syndesmophytes and eventual fusion of the spine. While tendons, cartilage, or bone marrow can be involved in the disease process, the defining and initial target is the enthesis.