A 3-week-old male infant with trisomy 21 presents with abdominal distension, bilious vomiting, and delayed passage of meconium. Contrast enema shows a cone-shaped transition zone with proximal megacolon and a narrow distal rectum. Anorectal manometry demonstrates absence of the rectoanal inhibitory reflex. A suction rectal biopsy confirms absence of ganglion cells with hypertrophied nerve trunks and strong acetylcholinesterase staining. After surgical planning, the resected specimen demonstrates the classic appearance of Hirschsprung disease. The structure marked **A** in the diagram—the narrow contracted distal segment—represents the aganglionic portion. Which of the following best explains the pathophysiological consequence of the anatomical abnormality at this site?
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