Comparing Early Versus Elective Colonoscopy

Inclusion Criteria: 1. Males or females aged ≥ 20 years, presenting with moderate-to-severe hematochezia or melena within 24 h of arrival, defined as (i) more than three occurrences of hematochezia within 8 h, (ii) hemorrhagic shock, or (iii) requiring transfusion. 2. Provision of signed and dated informed consent form. 3. Stated willingness to comply with all study procedures and availability for the duration of the study. Exclusion Criteria: An individual who meets any of the following criteria will be excluded from participation in this study: 1. Patients with hematemesis, black vomiting, or melena. 2. Patients with upper gastrointestinal bleeding, diagnosed by nasogastric tube or upper endoscopy. 3. Patients who have impossible consumed the oral bowel preparation solution. 4. Patients who have undergone computed tomography. 5. Patients who have been diagnosed with peptic ulcer diseases within the previous 10 days. 6. Ulcerative colitis or Crohn's disease patients. 7. Patients who have undergone abdominal surgery within the previous 10 days. 8. Patients who have undergone polypectomy, endoscopic mucosal resection, or endoscopic submucosal dissection of the lower gastrointestinal tract within the previous 10 days. 9. Patients with suspected perforation or peritonitis. 10. Patients with suspected intestinal obstruction. 11. Patients with hemorrhagic shock refractory to infusion or blood transfusion. 12. Patients who have undergone total colectomy. 13. Patients with suspected disseminated intravascular coagulation. 14. Patients with end-stage malignant disease. 15. Patients with severe cardiac failure. 16. Patients with active thrombosis. 17. Patients with severe respiratory failure. 18. Pregnant patients.