**Core Concept**
The question is testing the knowledge of a common cause of rectal bleeding in toddlers, which is often related to the anatomy and physiology of the lower gastrointestinal tract. The correct answer is related to a condition that affects the rectum and is characterized by the passage of blood in the stool.

**Why the Correct Answer is Right**
The correct answer is **D. Intussusception**. Intussusception occurs when one segment of the intestine invaginates into another, often causing ischemia and necrosis of the involved segment. In children, the most common site for intussusception is at the ileocecal junction. The presentation of intussusception is often classic, with the child experiencing severe abdominal pain, vomiting, and a "currant jelly" stool due to the presence of blood and mucous. The exact cause of intussusception is not always known, but it is thought to be related to an abnormal peristaltic wave or a lead point such as a Meckel's diverticulum.

**Why Each Wrong Option is Incorrect**
* **Option A:** Anal fissure is a tear in the anal mucosa and is a common cause of painful defecation, but it is not typically associated with passage of blood per rectum in toddlers.
* **Option B:** Hemorrhoids are a common cause of rectal bleeding in adults, but they are rare in children.
* **Option C:** Rectal polyps are a rare cause of rectal bleeding in children.

**Clinical Pearl / High-Yield Fact**
Intussusception is a medical emergency that requires prompt diagnosis and treatment to prevent bowel ischemia and necrosis.

**Correct Answer: D. Intussusception**