**Core Concept**
Congenital mega colon, also known as Hirschsprung's disease, is a congenital disorder characterized by the absence of ganglion cells in the distal colon and rectum, leading to a functional obstruction. This condition results from an embryonic developmental abnormality during the migration of neural crest cells to the hindgut.

**Why the Correct Answer is Right**
The diagnosis of congenital mega colon is confirmed by the presence of aganglionosis, which can be demonstrated by a rectal biopsy showing the absence of ganglion cells. This is typically assessed using a specialized staining technique, such as the acetylcholinesterase (AChE) stain, which highlights the presence of nerve fibers. The absence of AChE-positive nerve fibers in the submucosal and myenteric plexus confirms the diagnosis of Hirschsprung's disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while barium enema may show signs suggestive of congenital mega colon, such as a transition zone and dilated colon, it is not diagnostic.
* **Option B:** This option is incorrect because while ultrasound may show dilatation of the colon, it cannot confirm the diagnosis of congenital mega colon.
* **Option C:** This option is incorrect because while a colonoscopy may show signs suggestive of congenital mega colon, such as a transition zone and dilated colon, it is not diagnostic.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of congenital mega colon is the presence of a transition zone between the normal and affected segments of the colon. This transition zone is characterized by a narrowing of the lumen and a change in the caliber of the colon.

**Correct Answer:** D