**Core Concept:** Hirschsprung disease is a congenital gastrointestinal disorder characterized by the absence of ganglion cells in the distal portion of the colon, leading to impaired transit and constipation. This condition is due to defective neural crest cell migration during embryonic development.

**Why the Correct Answer is Right:** Hirschsprung disease is diagnosed using a series of tests to evaluate the patient's colon function. The gold standard test is the **double contrast barium enema** (DCE). This involves injecting barium sulfate contrast medium into the rectum and observing the mucosal folds and the transit of the contrast medium through the colon. The DCE is able to visualize the colon and detect any segmental or global aganglionic or hypoganglionic regions.

**Why Each Wrong Option is Incorrect:**

A. **Anorectal manometry (ARM)** measures the pressure within the rectum and anus. Although ARM can provide information about the rectum, it is not the definitive test for Hirschsprung disease diagnosis as it does not assess the colon.

B. **Sudden onset diarrhea and abdominal distension** are symptoms of Hirschsprung disease but are not diagnostic tests.

C. **Barium enema** is a radiographic examination that shows the anatomy of the colon, but it does not specifically evaluate colon transit and ganglion cell absence like DCE does.

D. **Cord sampling** involves examining the rectal biopsies for the presence of ganglion cells. Although this test is used to confirm the diagnosis, it is not the primary diagnostic test for Hirschsprung disease. Primary diagnosis is usually established using DCE, followed by cord sampling.

**Clinical Pearl:** Hirschsprung disease should be suspected in patients presenting with **constipation, abdominal distension, and failure to pass meconium** within the first month of life. Early diagnosis and treatment are crucial to prevent complications like megacolon, volvulus, and intestinal obstruction.

**Correct Answer:** D. **Cord sampling** involves examining rectal biopsies for the presence of ganglion cells. This test confirms the diagnosis of Hirschsprung disease, but the primary diagnostic test is usually **double contrast barium enema** (DCE). DCE reveals the aganglionic or hypoganglionic regions, confirming the suspicion of Hirschsprung disease. Cord sampling is useful for confirming the diagnosis, but DCE is the initial test to establish the clinical suspicion.