A 28-year-old man presents with symptoms of frequent bowel movements, crampy abdominal pain, and the passage of mucus. There is no history of any bloody diarrhea, but recently, he developed joint discomfort in his hands, knees, and back. On examination he is thin, and his abdomen is soft with voluntary guarding in the left lower quadrant. His joints are not actively inflamed and there is an asymmetric distribution. Which of the following is the most likely diagnosis?