Okay, let's tackle this question about Hirschsprung's disease. The user wants the investigation of choice. First, I need to recall what Hirschsprung's is. It's a congenital condition where there's an absence of ganglion cells in the myenteric plexus of the colon, leading to functional obstruction. The main issue is the aganglionic segment, which can cause severe constipation and bowel obstruction.

Now, the question is about the best investigation. From what I remember, the gold standard is a rectal biopsy. But wait, there are different types of biopsies. A full-thickness biopsy is needed because the ganglion cells are located in the myenteric plexus, which is between the muscular layers. A suction biopsy might not get enough tissue. So the correct answer is probably rectal suction biopsy.

Let me check the options. The user hasn't provided them, but standard options include barium enema, plain X-ray, MRI, and biopsy. So the correct answer would be the biopsy.

Now, the other options: Barium enema can show the transition zone but isn't diagnostic. It's more of an initial imaging test. MRI is used in some cases but not the first-line. Anorectal manometry might show lack of relaxation but isn't specific enough. So the biopsy is the investigation of choice.

Clinical pearl: Always remember that the biopsy must be full-thickness to visualize the myenteric plexus. Also, in some cases, a rectal manometry can support the diagnosis but can't confirm it alone.

**Core Concept**
Hirschsprung's disease is a congenital absence of ganglion cells in the myenteric plexus of the colon, causing functional obstruction. The diagnostic gold standard identifies the aganglionic segment through histopathological confirmation.

**Why the Correct Answer is Right**
The rectal suction biopsy is the investigation of choice. It allows direct visualization of the myenteric plexus (Auerbach’s plexus) for absence of ganglion cells. A full-thickness biopsy is required to assess both the submucosa and muscularis propria, ensuring accurate detection of aganglionosis. This method has high specificity and sensitivity compared to imaging or manometry.

**Why Each Wrong Option is Incorrect**
**Option A:** *Barium enema* may show a transition zone and "tapered colon" appearance but cannot confirm aganglionosis.
**Option B:** *Anorectal manometry* detects lack of rectoanal inhibitory reflex but is not diagnostic alone.
**Option C:** *Plain abdominal X-ray* is non-specific and used for acute obstruction, not diagnosis.
**Option D:** *MRI* is useful in complex cases but not the first-line test.

**Clinical Pearl / High-Yield Fact**
Always confirm Hirschsprung’s with a **rectal suction biopsy** (not superficial biopsy!). Remember: "Biopsy the bulb, not the tip" to ensure sampling of the myenteric plexus. Barium enema findings are suggestive but not definitive.

**Correct Answer: C. Rectal suction biopsy**

✓ Correct Answer: C. Rectal biopsy