**Question:** Investigation of choice in Hirschsprung's disease is

A. Barium enema
B. Methacholine Challenge Test
C. Anorectal manometry
D. Videofluoroscopy

**Core Concept:** Hirschsprung's disease is a congenital disorder characterized by the absence of ganglion cells in the distal part of the colon, leading to impaired motility and constipation.

**Why the Correct Answer is Right:** The correct answer is **C.** Anorectal manometry, also known as colonic manometry, is the most appropriate investigation for diagnosing Hirschsprung's disease. This test measures the pressure within the rectum and anal canal, which helps determine if there is abnormal low-pressure zone (i.e., Hirschsprung's disease).

**Why Each Wrong Option is Incorrect:**

A. Barium enema: This test visualizes the colonic wall contrast on X-ray but does not assess the rectal and anal motility, which is crucial for diagnosing Hirschsprung's disease.
B. Methacholine Challenge Test: This test assesses the response of the respiratory system to methacholine, not relevant to diagnosing Hirschsprung's disease.
D. Videofluoroscopy: This test evaluates swallowing function using X-rays, not relevant to diagnosing Hirschsprung's disease.

**Why the Correct Answer is Right:** Anorectal manometry, or colonic manometry, is the gold standard diagnostic test for Hirschsprung's disease. It helps assess rectal and anal motility, which is crucial for diagnosing this condition.

**Clinical Pearl:** Hirschsprung's disease diagnosis involves a combination of clinical history, physical examination, and appropriate investigations like anorectal manometry, rectal biopsy, and sometimes barium enema to demonstrate the transition zone. Identifying the absence of ganglion cells in the biopsy is definitive for diagnosis.