Ulcerative colitis is characterized by continuous mucosal inflammation starting in the rectum and extending proximally in the colon. This inflammation leads to ulceration and bleeding, presenting clinically as bloody diarrhea. Unlike Crohn’s disease, ulcerative colitis affects only the mucosal layer and exhibits continuous involvement without skip lesions. The presence of bloody diarrhea is a hallmark symptom that helps differentiate it from other conditions like irritable bowel syndrome or celiac disease, which do not typically cause mucosal inflammation or bleeding.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 21st Edition.