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 and is uncommon at 1 month of age 2
• The rectum would be distended and full of stool on examination, not empty 2
• Functional constipation typically shows onset after the neonatal period and lacks the delayed meconium passage history 1

Spinal Muscular Atrophy (Option C) - Not Supported

• Would present with generalized hypotonia, muscle weakness, and absent reflexes, none of which are described in this case
• Constipation would be a secondary feature, not the primary presentation

Hypothyroidism (Option D) - Unlikely

• While hypothyroidism can cause constipation, it would present with additional features such as prolonged jaundice, poor feeding, lethargy, and umbilical hernia
• The examination describes the infant as appearing well, inconsistent with untreated congenital hypothyroidism

Hypocalcemia (Option E) - Not Consistent

• Would present acutely with neuromuscular irritability, seizures, or tetany
• Does not cause chronic constipation since birth

Diagnostic Approach

Clinical Predictors of Hirschsprung's Disease

The presence of one or more of the following features identifies all patients with Hirschsprung's disease 1:

• Delayed passage of meconium (present in this case)
• Abdominal distension (present in this case)
• Vomiting (not mentioned in this case)
• Transition zone on contrast enema (not yet performed)

This patient has 2 of these 4 key features, making Hirschsprung's disease highly likely 1

Definitive Diagnosis Required

• Rectal biopsy is the definitive diagnostic test, showing absence of ganglion cells in the submucosal and myenteric plexuses 3, 4
• Contrast enema may show a transition zone and should be performed as part of the workup 1
• Without treatment, mortality approaches 50% by one year of age, making early diagnosis critical 3

Common Pitfall to Avoid

Do not dismiss constipation in a neonate as functional or benign—constipation present from birth or beginning in the neonatal period is most likely congenital in origin 2. The empty rectum on examination is the key clinical clue that should prompt immediate further evaluation for Hirschsprung's disease rather than empiric treatment for functional constipation.