**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. This results from a defective migration of neural crest cells during embryonic development, causing a failure of the enteric nervous system to form.

**Why the Correct Answer is Right**
The correct answer, **b) Absence of ganglia in involved segment**, is the underlying cause of Hirschsprung's disease. The absence of ganglion cells in the affected segment of the colon leads to a failure of the intestinal muscles to relax and pass stool, resulting in a functional obstruction. This absence of ganglia is due to the defective migration of neural crest cells during embryonic development. The affected segment is typically aganglionic, with normal ganglion cells present in the proximal colon.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hirschsprung's disease can present at any age, but it is most commonly diagnosed in infants and children. However, it can also be diagnosed in adults.
* **Option C:** The involved segment is not dilated in Hirschsprung's disease. Instead, it is typically narrow and contracted due to the absence of ganglion cells.
* **Option D:** While bleeding per rectum (PR) can be a presenting feature of Hirschsprung's disease, it is not the most common presenting symptom. The most common presenting symptoms are failure to pass meconium, abdominal distension, and constipation.

**Clinical Pearl / High-Yield Fact**
The "rectal biopsy" is a crucial diagnostic tool for Hirschsprung's disease. It involves examining a tissue sample from the rectum for the absence of ganglion cells. A "transition zone" can be seen where the aganglionic segment meets the normal ganglionic segment.

**Correct Answer:** b. Absence of ganglia in involved segment