A 3-day-old full-term male infant presents with abdominal distention and failure to pass meconium. Contrast enema shows a transition zone between a dilated proximal segment and a narrow distal bowel. Rectal suction biopsy 2 cm above the dentate line reveals absence of ganglion cells in both the myenteric and submucosal plexuses. The narrow contracted distal segment marked **C** in the diagram is aganglionic. Which of the following best explains the pathophysiology of functional obstruction in the segment marked **C**?

Explanation

## Why "Absence of ganglion cells prevents relaxation of the affected segment, resulting in tonic contraction and functional obstruction" is right The aganglionic segment marked **C** lacks ganglion cells of the myenteric (Auerbach) and submucosal (Meissner) plexuses due to failure of neural crest cell migration during weeks 5–12 of gestation. Without these ganglion cells, the affected bowel cannot relax in response to proximal peristalsis; instead, it remains in tonic contraction. This creates a functional obstruction—the narrow, contracted distal segment acts as an obstructing zone while the proximal bowel dilates (megacolon) in response. This is the pathophysiologic hallmark of Hirschsprung disease as described in Schwartz and Sabiston. ## Why each distractor is wrong - **Excessive parasympathetic innervation causes unopposed acetylcholine release and sustained muscle contraction**: This misrepresents the pathology. Hirschsprung disease is characterized by *absence* of ganglion cells (which mediate relaxation), not excess parasympathetic activity. The problem is loss of inhibitory innervation, not gain of excitatory innervation. - **Failure of smooth muscle development in the distal bowel leads to mechanical obstruction**: Hirschsprung disease is a disorder of the *enteric nervous system*, not smooth muscle development. Histology shows normal smooth muscle but absent ganglion cells. The obstruction is functional (neurogenic), not mechanical due to muscle hypoplasia. - **Increased blood supply to the distal segment impairs normal peristaltic function**: Vascular abnormalities are not part of the pathophysiology of Hirschsprung disease. The defect is purely neurogenic—absence of the enteric plexuses. **High-Yield:** Hirschsprung disease = absent ganglion cells → loss of relaxation → tonic contraction of distal segment → functional obstruction + proximal megacolon. Always starts at the anus and extends proximally. [cite: Schwartz Principles of Surgery 11e Ch 39; Sabiston Textbook of Surgery 21e Ch 68]