**Core Concept**
Hirschsprung's disease is a congenital condition characterized by the absence of ganglion cells in the distal colon, leading to a functional obstruction due to the failure of the affected segment to relax and pass stool.

**Why the Correct Answer is Right**
The absence of ganglion cells in Hirschsprung's disease is due to a mutation in the RET proto-oncogene, which is crucial for the development of the enteric nervous system. This results in an aganglionic segment of the colon that fails to relax during defecation, causing a functional obstruction. The affected segment typically starts from the rectum and extends proximally for varying distances.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Hirschsprung's disease is not primarily caused by a viral infection.
**Option B:** This option is incorrect because the condition is not characterized by an overactive colon.
**Option C:** This option is incorrect because the condition is not primarily caused by a mechanical obstruction.

**Clinical Pearl / High-Yield Fact**
A key clinical feature of Hirschsprung's disease is the presence of a "transition zone" between the aganglionic and normal colon segments, which can be identified on a barium enema.

**Correct Answer:** D