**Core Concept**
Hirschsprung's disease is a congenital condition characterized by the absence of ganglion cells in the distal colon and rectum, leading to a functional obstruction. The absence of these neurons results in a segment of the colon that is unable to relax and pass stool, causing a buildup of feces proximal to the affected area.

**Why the Correct Answer is Right**
In Hirschsprung's disease, the aganglionic segment is the part of the colon that lacks ganglion cells. This segment is typically found in the rectosigmoid region and is distal to the dilated segment, which is the portion of the colon that becomes enlarged due to the accumulation of stool. The absence of ganglion cells in this area leads to a failure of peristalsis, resulting in the characteristic clinical presentation of the disease. The affected segment is usually identified by the presence of a transition zone between the aganglionic and normal colon.

**Why Each Wrong Option is Incorrect**
**Option B:** The whole colon is not affected in Hirschsprung's disease, as the aganglionic segment is typically limited to the rectosigmoid region.
**Option C:** The aganglionic segment is not found proximal to the dilated segment, as the dilated segment is the portion of the colon that is affected by the accumulation of stool proximal to the aganglionic area.
**Option D:** The aganglionic segment is not found in the dilated segment, as the dilated segment is the portion of the colon that is affected by the accumulation of stool, whereas the aganglionic segment is the area that lacks ganglion cells.

**Clinical Pearl / High-Yield Fact**
In Hirschsprung's disease, the transition zone between the aganglionic and normal colon is often characterized by a change in the caliber of the colon, with the aganglionic segment being narrower than the normal segment. This transition zone is an important landmark for surgical intervention.

✓ Correct Answer: A. Distal to dilated segment