Okay, let's tackle this question about the investigation of choice for Hirschsprung's disease. First, I need to recall what Hirschsprung's disease is. It's a congenital condition where a part of the large intestine lacks the enteric nervous system, leading to functional obstruction. The main issue is the absence of ganglion cells in the myenteric and submucosal plexuses of the distal colon. So, the diagnosis usually involves identifying this aganglionosis.

The options weren't provided, but the correct answer is supposed to be one of the standard investigations. From what I remember, the gold standard for diagnosing Hirschsprung's disease is a rectal biopsy. There are two types: full-thickness biopsy and suction biopsy. The full-thickness biopsy is considered more accurate because it samples both layers of the colon wall where the ganglion cells are located. The suction biopsy might miss the aganglionic segment if the sample isn't taken from the transitional zone.

Wait, but sometimes there's a mention of other imaging studies like abdominal X-rays or barium enema, but those are more for initial assessment or to check for complications like enterocolitis. The actual confirmation comes from histological examination of the tissue. So the correct answer here should be rectal biopsy. Let me check if there are other options. Sometimes, some sources might list anorectal manometry, but that's more for evaluating functional disorders like constipation, not Hirschsprung's specifically. Anorectal manometry can show a lack of relaxation of the internal anal sphincter in Hirschsprung's, but it's not diagnostic on its own. The definitive test is the biopsy.

So the core concept here is that histological confirmation via rectal biopsy is the gold standard. The options might include other tests like barium enema, manometry, or imaging, but they're not as definitive. The explanation should highlight why the biopsy is the correct choice and why others are incorrect. Also, a clinical pearl would be to remember that biopsy is necessary for confirmation, even though other tests can suggest the diagnosis.

**Core Concept**
Hirschsprung’s disease is diagnosed by identifying **absent ganglion cells** in the **myenteric and submucosal plexuses** of the distal colon. The **rectal biopsy** is the gold standard to confirm this histologically.

**Why the Correct Answer is Right**
A **full-thickness rectal biopsy** (Option C) is the definitive diagnostic test. It samples both the muscularis externa and submucosa, allowing histopathological confirmation of aganglionosis. The biopsy is typically taken from the **transition zone** (between aganglionic and ganglionic bowel) to ensure accuracy. This method avoids sampling error and provides a conclusive diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** *Barium enema* may show a “transition zone” or “lead point,” but these are indirect signs and cannot confirm the diagnosis.
**Option B:** *Anorectal manometry* detects impaired rectoanal inhibitory reflex but is not diagnostic alone and is used preoperatively to assess sphincter function.
**Option D:** *Abdominal X-ray* may suggest constipation or obstruction but lacks specificity for Hirschsprung’s.

✓ Correct Answer: C. Rectal biopsy