A 50-year-old man has longstanding ulcerative colitis diagnosed at age 20 years is presently asymptomatic and is receiving maintenance treatment with infliximab and 6-mercaptopurine. His bowel function is stable, at 6 nonbloody stools per day without any associated complaints. He undergoes colonoscopy which demonstrates minimal evidence of inflammation. Random biopsies reveal low-and high-grade dysplasia 2 cm, 30 cm, and 50 cm from the anal verge. The remainder of the biopsies reveal no dysplasia. What is the best next step in management?