Ulcerative colitis. Light micrograph of a section through the inflamed mucosal lining of a human large intestine (colon). An ulcer (upper center) is a break in the lining of the skin or alimentary tract, which fails to heal and becomes inflamed. Ulcerative colitis has no known cause and can vary in severity from month to month. It can be treated with corticosteroid drugs and bed rest. If the condition is severe or extensive, surgical removal (colectomy) of part or all of the colon may be considered. Magnification unknown.