Esophagogastroduodenoscopy (EGD), also known as upper endoscopy, is a diagnostic procedure that involves the use of a flexible, lighted tube called an endoscope to examine the lining of the esophagus, stomach, and the upper part of the small intestine (duodenum). This procedure helps in diagnosing and treating various gastrointestinal conditions, such as ulcers, inflammation, tumors, and bleeding.