**Meckel's Diverticulum**

**Core Concept**
Meckel's diverticulum is a congenital anomaly resulting from the incomplete obliteration of the vitelline duct, a remnant of the omphalomesenteric duct. This condition is characterized by the presence of a small, bulging pouch on the antimesenteric border of the ileum. Meckel's diverticulum is a common cause of gastrointestinal bleeding in children and adults.

**Why the Correct Answer is Right**
Meckel's diverticulum contains ectopic gastric or pancreatic tissue, which can lead to ulceration and bleeding. The presence of gastric mucosa in the diverticulum can secrete acid and pepsin, causing erosion and bleeding of the surrounding intestinal tissue. This ectopic tissue can be a source of chronic or acute gastrointestinal bleeding, and its presence can be confirmed by endoscopy or surgical exploration.

**Why Each Wrong Option is Incorrect**
**Option A:**
Meckel's diverticulum is not typically associated with intussusception, which is a condition where one segment of intestine protrudes into another. Intussusception is more commonly caused by a lead point such as a tumor or a polyp.

**Option B:**
Meckel's diverticulum is not typically associated with appendicitis. While both conditions involve the appendix and the ileum, they are distinct entities with different pathologies.

**Option C:**
Meckel's diverticulum is not typically associated with Crohn's disease. While both conditions involve the small intestine, they have distinct pathologies and clinical presentations.

**Clinical Pearl / High-Yield Fact**
Meckel's diverticulum is a common cause of gastrointestinal bleeding in children and adults, and its presence can be confirmed by endoscopy or surgical exploration.

**Correct Answer:** D.