This study focuses on people with ulcerative colitis (an inflammatory disease of the bowel) who are starting a drug called etrasimod as standard of care. The study aims to show that etrasimod reduces abnormal blood flow in the inflamed colon in these patients. Researchers also want to determine whether changes in blood flow in the colon can predict how well ulcerative colitis patients respond to etrasimod after 12 weeks. Study procedures include blood and stool sample collection, questionnaires, medication review, and intestinal ultrasound to monitor blood flow.