**Core Concept**
The newborn's symptoms of bloated abdomen and reduced meconium passage are suggestive of a congenital obstruction or motility disorder of the gastrointestinal tract. A rectal biopsy is a diagnostic tool used to assess the rectal mucosa for specific pathological features.

**Why the Correct Answer is Right**
The most likely diagnosis in this scenario is Hirschsprung's disease, a congenital condition characterized by the absence of ganglion cells in the distal rectum and sigmoid colon. This results in a functional obstruction, leading to the accumulation of stool and gas in the colon, causing abdominal distension. The full-thickness biopsy of the rectum would reveal the absence of ganglion cells in the submucosal and myenteric plexuses, which is a hallmark of Hirschsprung's disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a diagnostic feature of Hirschsprung's disease. While it may be a feature of other conditions, it is not the most likely finding in this scenario.
* **Option B:** This option is incorrect because it describes a normal finding. The presence of ganglion cells in the rectal biopsy is expected in a healthy individual.
* **Option C:** This option is incorrect because it describes a feature of other conditions, such as inflammatory bowel disease. While it may be present in some cases, it is not the most likely finding in this scenario.
* **Option D:** This option is incorrect because it does not specify a diagnostic feature of Hirschsprung's disease. While it may be a feature of other conditions, it is not the most likely finding in this scenario.

**Clinical Pearl / High-Yield Fact**
In Hirschsprung's disease, the absence of ganglion cells in the rectal biopsy is a diagnostic hallmark. This condition requires prompt surgical intervention to prevent complications such as intestinal obstruction and perforation.

**Correct Answer:** C. Aganglionosis of the distal rectum