**Core Concept**
The most common cause of severe rectal bleeding in the pediatric age group is related to an aberrant pancreatic or gastric mucosa located within an ileal diverticulum. This is due to the ectopic tissue secreting acid and leading to ulceration and bleeding.

**Why the Correct Answer is Right**
The ileal (Meckel) diverticulum is a congenital anomaly that occurs due to a failure of the vitelline duct to close during embryonic development. Approximately 2-3% of the population has a Meckel's diverticulum, and it is a common cause of lower gastrointestinal bleeding in children. The presence of ectopic gastric or pancreatic mucosa within the diverticulum leads to acid production, which causes ulceration and subsequent bleeding. This condition typically presents with painless, profuse rectal bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** Internal hemorrhoids are a common cause of rectal bleeding in adults but are rare in children. They are typically associated with chronic constipation and are not usually painful.

**Option B:** External hemorrhoids can cause bleeding, but it is usually mild and not profuse. They are also not typically associated with painless rectal bleeding.

**Option C:** Diverticulosis is a condition characterized by the presence of diverticula in the colon wall. While it can cause bleeding, it is more commonly seen in adults and is less likely to be the cause of severe rectal bleeding in children.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is a classic example of a congenital anomaly that can cause significant morbidity in children. It is essential to consider this condition in the differential diagnosis of pediatric gastrointestinal bleeding.

**✓ Correct Answer: D. Ileal (Meckel) diverticulum**