<s>[INST] context: A Meckel's diveiculum is caused by failure of obliteration of the vitello-intestinal duct. It is classically located in the distal ileum within 30 cm of the ileocecal valve, and the structure is a true diveiculum with mucosa, submucosa, and muscularis propria. Many Meckel's diveicula contain ectopic pancreatic tissue or gastric mucosa, and the acid production from the gastric mucosa may be sufficient to produce a small peptic ulcer in adjacent intestinal mucosa. Such small peptic ulcers are occasional sources of mysterious appendicitis-like pain or intestinal bleeding. Peptic ulceration adjacent to a Meckel's diveiculum should be suspected in any child who presents with massive, painless rectal bleeding. Technetium concentrates in gastric mucosa, and the scan in this patient demonstrated a small amount of ectopic gastric mucosa located in the diveiculum.Acute appendicitis is usually very painful and does not typically cause rectal bleeding. A lesion of the cecum would have been revealed by thorough colonoscopy. Failure of upper endoscopy to demonstrate a peptic ulcer of the duodenum makes duodenal disease unlikely. [/INST] question: A 5-year-old child is brought to the emergency room with massive, painless bleeding from the rectum. Colonoscopy fails to demonstrate a lesion in the colon or anus. Upper endoscopy fails to demonstrate esophagitis, gastric ulcer or duodenal ulcer. A 99mTc (technetium) scan demonstrates an abnormality in the lower half of the abdomen. Failure of a normal developmental process involving which of the following structures is the most likely cause of this child's bleeding? | options: A: Appendix B: Cecum C: Duodenum D: Ileum | answer: D </s>