A 37-year-old woman has chronic diarrhea for 2 years, with 2-3 bowel movements daily, often containing small amounts of mucus. Repeated stool pathogen cultures are negative. Colonoscopy reveals congestion, edema, and a few scattered superficial ulcers in the rectum, descending colon, and transverse colon. The diagnosis is ulcerative colitis. The first-line treatment is: