Ulcer - A lesion on the surface of the skin or a mucous membrane characterized by a superficial loss of tissue.

Ulcers are most common on the skin of the lower extremities and in the gastrointestinal tract, although they may be encountered at almost any site.

The diverse causes of ulcers range from circulatory disturbances or bacterial infections to complex, multifactorial disorders. The superficial tissue sloughs, leaving a crater that extends into the underlying soft tissue, which then becomes inflamed and is subject to further injury by the original offender or secondary infection.

Peptic ulcer is the most common ulcer of the gastrointestinal tract and refers to breaks in the mucosa of the stomach or the proximal duodenum that are produced by the action of gastric secretions. It is still unknown why peptic ulcers develop. However, with rare exceptions, a person who does not secrete hydrochloric acid will not develop a peptic ulcer. Ulcers of the stomach tend to develop as a result of superficial inflammation of the stomach. These individuals tend to have normal or decreased amounts of hydrochloric acid. By contrast, most individuals with peptic ulcers of the proximal duodenum secrete excessive amounts of acid. Importantly, a bacterium, Helicobacter pylori, has been isolated from the stomach of most people with peptic ulcers and is thought to play a causative role. Although stress has been anecdotally related to peptic ulcers for at least a century, serious doubt has been cast upon this concept.

Ulcerative colitis is a disease of the large intestine characterized by chronic diarrhea and rectal bleeding. The disorder is common in the Western world, occurring principally in young adults. Its cause is not known, but there is some evidence for a familial predisposition to the disease.