**Core Concept**
Hirschsprung's disease is a congenital condition characterized by the absence of ganglion cells in the distal colon and rectum. This results in a functional obstruction due to the failure of the affected segment to relax and allow stool to pass. The diagnosis is typically confirmed by a rectal biopsy.

**Why the Correct Answer is Right**
In Hirschsprung's disease, the rectal biopsy reveals an absence of ganglion cells in the submucosal and myenteric plexuses. The presence of nerve trunks and the absence of ganglion cells are characteristic features. The biopsy also shows an increase in the number of nerve trunks due to the compensatory hypertrophy of the remaining nerve fibers. This is a result of the absence of inhibitory neurons that normally regulate the contraction and relaxation of the intestinal muscles.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Hirschsprung's disease is not characterized by an increase in the number of ganglion cells. The absence of ganglion cells is a hallmark of the disease.
**Option B:** This option is incorrect because the rectal biopsy in Hirschsprung's disease does not show an increase in the number of nerve trunks. While there is an increase in the number of nerve trunks due to compensatory hypertrophy, this is not the primary feature of the disease.
**Option C:** This option is incorrect because the rectal biopsy in Hirschsprung's disease does not show an increase in the number of smooth muscle cells. The disease is characterized by the absence of ganglion cells, not an increase in smooth muscle cells.

**Clinical Pearl / High-Yield Fact**
The absence of ganglion cells in the rectal biopsy is a reliable diagnostic feature of Hirschsprung's disease. This can be confirmed by the presence of nerve trunks and the absence of ganglion cells in the submucosal and myenteric plexuses.

**Correct Answer:** D