What is the most common extraintestinal manifestation of ulcerative colitis and Crohn's disease?

Approximately 20% of patients have arthritis, either symmetric sacroiliitis and spondylitis, such as that seen in AS, or an oligoarticular peripheral arthritis of the lower limbs. Typically, the arthritis is not deforming and parallels the activity of the inflammatory bowel disease in exacerbation and remission.