Esophago-gastro-duodenoscopy — usually shortened to OGD or upper endoscopy — is the diagnostic medical procedure in which an endoscope is passed through the alimentary tract’s upper reaches: down the oesophagus, through the stomach, and into the duodenum. Under sedation, the flexible instrument inspects the lining for ulcers, inflammation, coeliac damage, tumours, and sources of bleeding, taking biopsies where needed and treating as it goes — clipping bleeders, dilating strictures, removing polyps. For nutrition practice it provides the definitive answers: the duodenal biopsy that confirms coeliac disease, the gastritis that explains iron deficiency, the varices that rewrite a liver patient’s diet. Preparation means fasting; the procedure itself takes minutes. It is direct vision applied to the gut’s busiest stretch — diagnosis at the source.