**Core Concept**
Intussusception is a medical emergency characterized by the telescoping of one segment of the intestine into another, leading to bowel obstruction and potentially severe complications. The most common type of intussusception is a critical concept in surgical management.

**Why the Correct Answer is Right**
The most common type of intussusception is ileocolic, where the ileum (the last part of the small intestine) invaginates into the colon. This is often due to a lead point, such as a Meckel's diverticulum, a polyp, or a lymphoid hyperplasia, which initiates the intussusception. Ileocolic intussusception typically presents with abdominal pain, vomiting, and bloody stools, and requires prompt surgical intervention to prevent bowel ischemia and necrosis.

**Why Each Wrong Option is Incorrect**
**Option B:** Colocolic intussusception, where one part of the colon invaginates into another, is less common and often associated with a malignant lead point.
**Option C:** Ileoileal intussusception, where one part of the small intestine invaginates into another, is rare and usually seen in children with a history of previous abdominal surgery.
**Option D:** Retrograde intussusception is not a recognized type of intussusception and is likely a distractor.

**Clinical Pearl / High-Yield Fact**
The classic triad of intussusception is abdominal pain, vomiting, and bloody stools, but the presence of a palpable abdominal mass and a history of recent viral illness can also suggest this diagnosis.

**✓ Correct Answer: A. Ileocolic**