A 38-year-old man with a 12-year history of ulcerative colitis presents with a 3-week history of bloody diarrhea (6–8 stools/day), fever (38.2°C), and abdominal cramping. Colonoscopy is performed and reveals the pattern marked **A** in the diagram—continuous circumferential inflammation extending from the rectum proximally to the splenic flexure with a sharp demarcation to normal mucosa. Which of the following BEST characterizes the pathophysiologic basis of this continuous, symmetric inflammatory pattern in ulcerative colitis?