**Core Concept**
Hirschsprung disease is a congenital condition characterized by the absence of ganglion cells in the distal colon and rectum, leading to functional obstruction. This condition is caused by a failure of neural crest cells to migrate during embryonic development.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of Hirschsprung disease. Hirschsprung disease is indeed characterized by the absence of ganglion cells, which are crucial for peristalsis and intestinal motility. This absence leads to a functional obstruction, as the affected segment of the colon is unable to relax and pass stool. The affected segment becomes dilated and hypertrophic, while the proximal segment becomes redundant and hypertrophic due to the accumulation of stool.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Hirschsprung disease is not caused by a viral infection. The condition is a congenital defect resulting from the failure of neural crest cells to migrate during embryonic development.
* **Option B:** This option is incorrect because Hirschsprung disease is not typically associated with an increase in the number of ganglion cells. Instead, the condition is characterized by the absence of ganglion cells in the affected segment of the colon.
* **Option C:** This option is incorrect because Hirschsprung disease is not a type of intussusception. While both conditions can cause intestinal obstruction, they have distinct pathophysiological mechanisms.

**Clinical Pearl / High-Yield Fact**
Hirschsprung disease is often associated with other congenital anomalies, including Down syndrome, Waardenburg syndrome, and congenital megacolon. A thorough evaluation for these conditions is essential in the diagnostic workup of Hirschsprung disease.

**Correct Answer: A. This option is incorrect because Hirschsprung disease is not caused by a viral infection.**