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?
A. Mucosal ulceration and stricture formation in the distal rectum result from chronic faecal stasis
B. Hypertrophy of the muscularis propria in the distal segment leads to mechanical obstruction independent of neural control
C. Excessive sympathetic innervation causes unopposed contraction of the distal rectum, preventing normal peristalsis
D. Absence of parasympathetic ganglion cells prevents relaxation of the internal anal sphincter and distal rectum, creating a functional obstruction
Explanation
Why "Absence of parasympathetic ganglion cells prevents relaxation of the internal anal sphincter and distal rectum, creating a functional obstruction" is right
The structure marked A—the narrow contracted aganglionic distal segment—lacks enteric ganglion cells in both submucosal and myenteric plexuses. These plexuses normally contain parasympathetic neurons that mediate relaxation (via nitric oxide and VIP) of the internal anal sphincter and distal rectum. Without these inhibitory neurons, the distal segment remains tonically contracted and cannot relax in response to proximal faecal boluses, creating a functional obstruction. This absence of the rectoanal inhibitory reflex (confirmed by anorectal manometry in this case) is the hallmark pathophysiology of Hirschsprung disease. The proximal colon then dilates (megacolon) as it attempts to overcome this fixed, non-relaxing distal obstruction. (Robbins and Cotran Pathologic Basis of Disease, 10th ed., Ch. The Gastrointestinal Tract.)
Why each distractor is wrong
Excessive sympathetic innervation causes unopposed contraction of the distal rectum, preventing normal peristalsis: Hirschsprung disease is characterized by absence of ganglion cells, not excessive sympathetic innervation. The problem is loss of inhibitory (parasympathetic) tone, not gain of excitatory (sympathetic) tone. Sympathetic fibres are often preserved in aganglionic segments.
Hypertrophy of the muscularis propria in the distal segment leads to mechanical obstruction independent of neural control: While the distal segment is indeed narrow and contracted, this is a functional consequence of absent neural relaxation, not primary muscular hypertrophy. The proximal segment shows muscular hypertrophy (megacolon), but the distal segment is characteristically thin-walled and contracted due to unopposed tonic contraction from loss of inhibitory innervation.
Mucosal ulceration and stricture formation in the distal rectum result from chronic faecal stasis: Mucosal ulceration (stercoral ulcers) occurs in the proximal dilated megacolon segment due to pressure necrosis from impacted faeces, not in the distal aganglionic segment. The distal segment is narrow and contracted, not ulcerated.
High-YieldNEET PG
Hirschsprung disease = absence of enteric ganglion cells in distal bowel → loss of inhibitory (parasympathetic) relaxation → functional obstruction at the aganglionic-ganglionic transition → proximal megacolon.
Robbins and Cotran Pathologic Basis of Disease, 10th ed., Ch. The Gastrointestinal Tract.
Practice similar questions
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.
