**Core Concept**
The child's symptoms are suggestive of rectal bleeding due to a condition called Meckel's diverticulum, which is a congenital anomaly resulting from the incomplete closure of the vitelline duct. This duct connects the yolk sac to the intestinal tract during embryonic development.

**Why the Correct Answer is Right**
Meckel's diverticulum is a true diverticulum, containing all layers of the intestinal wall, and is usually located on the antimesenteric border of the ileum. It often contains ectopic gastric or pancreatic tissue, which can lead to ulceration and bleeding. The symptoms in this case, including massive, painless rectal bleeding and failure of colonoscopy and upper endoscopy to demonstrate a lesion, are consistent with Meckel's diverticulum. The abnormality seen on the 99mTc scan is likely due to the ectopic gastric mucosa in the diverticulum, which can be visualized by this scan due to the uptake of technetium by the gastric mucosa.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a Meckel's diverticulum is a diverticulum of the ileum, not the colon. While a colon diverticulum could cause bleeding, the symptoms and findings in this case are more consistent with a Meckel's diverticulum.
**Option B:** This option is incorrect because an intussusception is a condition where a part of the intestine telescopes into another part, leading to bowel obstruction and bleeding. While intussusception can cause rectal bleeding, it would typically be accompanied by signs of bowel obstruction, such as abdominal pain and vomiting.
**Option C:** This option is incorrect because a Hirschsprung's disease is a congenital condition characterized by the absence of ganglion cells in the distal colon and rectum, leading to functional obstruction. While Hirschsprung's disease can cause rectal bleeding, the symptoms in this case are not consistent with this condition.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is the most common congenital anomaly of the small intestine and is often asymptomatic. However, it can cause significant bleeding, especially in children, due to the presence of ectopic gastric or pancreatic tissue.

**Correct Answer: C. Hirschsprung's disease is incorrect, but the missing option is likely "D. Vitelline duct anomaly". The correct answer is likely "D. Vitelline duct anomaly".