**Core Concept**
The "saw-tooth" appearance on a barium enema is a radiological finding associated with a specific gastrointestinal condition. It is characterized by a series of alternating mucosal folds and barium-filled structures, resembling the teeth of a saw.

**Why the Correct Answer is Right**
The "saw-tooth" appearance is typically seen in **Hirschsprung's disease**, a congenital condition characterized by the absence of ganglion cells in the distal colon and rectum. This leads to a functional obstruction, resulting in a characteristic narrowing of the colon and a "saw-tooth" appearance on barium enema due to the alternating mucosal folds and barium-filled structures. The absence of ganglion cells in the affected segment of the colon causes an accumulation of barium, leading to the characteristic radiographic appearance.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the "saw-tooth" appearance is not typically seen in **Meckel's diverticulum**, a congenital diverticulum of the ileum. While Meckel's diverticulum can cause gastrointestinal bleeding and obstruction, it does not typically cause a "saw-tooth" appearance on barium enema.

**Option B:** This option is incorrect as the "saw-tooth" appearance is not typically seen in **Crohn's disease**, a type of inflammatory bowel disease. While Crohn's disease can cause narrowing and stricture formation in the colon, it typically causes a more irregular and "cobblestone" appearance on barium enema, rather than a "saw-tooth" appearance.

**Option C:** This option is incorrect as the "saw-tooth" appearance is not typically seen in **Ulcerative Colitis**, a type of inflammatory bowel disease. While ulcerative colitis can cause inflammation and narrowing of the colon, it typically causes a more uniform and diffuse appearance on barium enema, rather than a "saw-tooth" appearance.

**Clinical Pearl / High-Yield Fact**
The "saw-tooth" appearance on barium enema is a classic radiographic finding in Hirschsprung's disease, and is often associated with a history of constipation and abdominal distension in infants and young children.

**Correct Answer: A. Hirschsprung's disease**