The formation of open sores or lesions on the mucous membranes lining the gastrointestinal tract. These ulcers arise when the protective mucus barrier breaks down, exposing underlying tissue to acid, enzymes, or inflammation, as in peptic ulcers, Crohn’s disease, ulcerative colitis, and severe infections. Symptoms include pain, bleeding visible in vomit or stools, and anaemia from chronic blood loss. Diagnosis uses endoscopy with biopsy, and treatment targets the cause: acid suppression, anti-inflammatory drugs, eradication of Helicobacter pylori, or management of inflammatory bowel disease. Nutrition support focuses on adequate protein and iron for healing while avoiding irritants during flares.