**Question:** Hirschsprung's disease is diagnosed by: March 2005

**Core Concept:** Hirschsprung's disease is a congenital disorder characterized by the absence of ganglion cells in the submucosa of the rectum and large intestine, leading to impaired peristalsis and fecal mass formation.

**Why the Correct Answer is Right:**
Hirschsprung's disease is diagnosed using a combination of clinical presentation, examination findings, and radiological and endoscopic investigations. In March 2005, the gold standard diagnostic test for Hirschsprung's disease was still the rectal suction biopsy, which involves obtaining a biopsy specimen from the rectal mucosa to evaluate the presence of ganglion cells. This test is specific and sensitive in diagnosing the condition.

**Why Each Wrong Option is Incorrect:**
A. Anorectal manometry: This test measures the pressure changes in the anus and rectum, which is not specific to Hirschsprung's disease diagnosis.
B. Barium enema: This radiological test shows abnormal colonic dilatation and lack of contrast passage, but it is not always diagnostic, especially in infants.
C. Methylene blue test: This test involves the administration of methylene blue dye and inspection for its passage, which is not a reliable diagnostic tool.
D. Neonatal clinical presentation: While clinical presentation can guide suspicion, it is not a definitive diagnostic test for Hirschsprung's disease.

**Clinical Pearl / High-Yield Fact:**
In infants and children presenting with constipation, abnormal bowel habits, or abdominal distension, Hirschsprung's disease should be considered, and clinical suspicion may guide the need for further diagnostic tests like rectal suction biopsy. However, the gold standard remains the rectal suction biopsy in 2005, as mentioned in the question. This test is necessary to confirm the diagnosis and rule out other differential diagnoses.