The presence of melena indicates upper gastrointestinal (GI) bleeding, typically from a source proximal to the ligament of Treitz. Upper GI endoscopy is the gold standard diagnostic tool as it allows direct visualization, diagnosis, and potential therapeutic intervention of bleeding lesions such as peptic ulcers or gastritis. Colonoscopy is less appropriate initially because melena usually does not originate from the lower GI tract. Abdominal ultrasound and CT abdomen are not first-line investigations for acute GI bleeding. Prompt upper GI endoscopy can reduce morbidity by enabling rapid diagnosis and management.

Reference: Sleisenger and Fordtran’s Gastrointestinal and Liver Disease, Feldman et al., 11th Edition.