What is the most likely diagnosis for a 1-month-old boy with constipation since birth, presenting with infrequent bowel movements, a mildly distended abdomen, and an empty rectum on examination?

Hirschsprung's Disease

The most likely diagnosis is Hirschsprung's disease (Option B), based on the constellation of delayed meconium passage, infrequent bowel movements since birth, absence of diaper soiling, and the critical finding of an empty rectum on examination in a 1-month-old infant.

Key Diagnostic Features Present in This Case

Classic Presentation Pattern

• Onset in the first week of life is seen in 60% of Hirschsprung's disease cases, compared to only 15% of functional constipation cases 1
• Delayed passage of meconium (passed motion two days after birth rather than within 24-48 hours) is a hallmark feature that significantly increases the likelihood of Hirschsprung's disease 1
• Infrequent bowel movements (every 4-5 days) without diaper soiling indicates true constipation rather than functional issues 2

Critical Physical Examination Finding

• Empty rectum on rectal examination is the pathognomonic finding that distinguishes Hirschsprung's disease from functional constipation 2
• In functional constipation, the rectum is typically distended and full of stool, whereas in Hirschsprung's disease the rectum is characteristically empty and tight 2
• The mildly distended abdomen reflects proximal stool accumulation above the aganglionic segment 1

Why Other Diagnoses Are Less Likely

Functional Constipation (Option A) - Excluded

• Functional constipation rarely presents in the neonatal period or first month of life 2
• The rectum would be distended and full of stool on examination, not empty 2
• Delayed meconium passage is uncommon in functional constipation (present in only 60% versus 98% in Hirschsprung's disease) 1

Spinal Muscular Atrophy (Option C) - Not Supported

• Would present with generalized hypotonia, muscle weakness, and absent deep tendon reflexes, none of which are described 2
• The child "appears well" on examination, inconsistent with this neuromuscular disorder

Hypothyroidism (Option D) - Unlikely

• Would typically present with additional features such as prolonged jaundice, poor feeding, lethargy, large fontanelles, and umbilical hernia 2
• The isolated presentation of constipation without systemic signs makes this diagnosis less probable

Hypocalcemia (Option E) - Not Consistent

• Would present acutely with neuromuscular irritability, seizures, or tetany rather than chronic constipation since birth 2
• The chronic nature of symptoms from birth argues against a metabolic derangement

Diagnostic Approach

Confirmatory Testing Required

• Rectal biopsy remains the definitive diagnostic test, demonstrating absence of ganglion cells in the submucosal and myenteric plexuses 3, 4
• Acetylcholinesterase histochemistry or calretinin immunohistochemistry can complement standard histology 4
• Contrast enema may show a transition zone but is not definitive 1

Clinical Predictors

The presence of delayed meconium passage, abdominal distension, vomiting, or transition zone on contrast enema identified 100% of Hirschsprung's disease patients in one study 1. This patient has at least two of these features (delayed meconium, abdominal distension).

Critical Clinical Pearls

• Without treatment, mortality approaches 50% by one year of age, making early diagnosis essential 3
• The classic triad (delayed meconium passage, vomiting, abdominal distension) is present in only 18% of cases, but one or more symptoms occur in 98% 1
• Surgical removal of the aganglionic segment is the definitive treatment 3, 5
• This case represents a typical presentation requiring urgent referral to pediatric surgery for rectal biopsy and surgical planning 4