Sacro-Iliac joint involvement is common in which condition?

(A) Ankylosing spondylitis # ANKYLOSING SPONDYLITIS (AS, from Greek ankylos, crooked; spondylos, vertebrae; -itis, inflammation), previously known as Bekhterev's disease, Bekhterev syndrome, and Marie-Strumpell disease, is a chronic inflammatory disease of the axial skeleton with variable involvement of peripheral joints and nonarticular structures. AS is a form of spondyloarthritis, a chronic, inflammatory arthritis where immune mechanisms are thought to have a key role. It mainly affects joints in the spine and the sacroiliac joint in the pelvis, and can cause eventual fusion of the spine.> The typical patient is a young male, aged 20-40; however, the condition also presents in females.> The condition is known to be hereditary. Symptoms of the disease first appear, on average, at age 23 years. These first symptoms are typically chronic pain and stiffness in the middle part of the spine or sometimes the entire spine, often with pain referred to one or other buttock or the back of thigh from the sacroiliac joint.