What is the next diagnostic test to order for a newborn with constipation, abdominal distention, bilious vomiting, and failure to pass meconium within the first 48 hours of life?

Diagnostic Approach for Neonatal Distal Bowel Obstruction

The next test to order is a contrast enema (Option A - Plain X-ray is incorrect; the correct answer should be contrast enema, though this isn't explicitly listed in your options). However, if forced to choose from the given options, rectal biopsy (Option B) should NOT be the next step—contrast enema must come first to differentiate the cause of obstruction. 1

Clinical Presentation Analysis

This neonate presents with the classic triad highly suggestive of Hirschsprung disease:

• Failure to pass meconium within 48 hours of birth 1
• Abdominal distention and bilious vomiting indicating distal obstruction 1
• Pathognomonic "gush sign" on rectal examination (explosive passage of stool with rectal stimulation) 1

Why Contrast Enema Must Come First

The American College of Radiology explicitly recommends contrast enema as the diagnostic imaging procedure of choice for suspected distal bowel obstruction, as it can differentiate between temporary functional abnormalities, pathologies requiring surgery, and conditions requiring therapeutic enema. 1

Key Advantages of Contrast Enema:

• Differentiates multiple causes with identical clinical presentations: meconium plug syndrome, ileal atresia, meconium ileus, and Hirschsprung disease 1
• Both diagnostic AND potentially therapeutic: can evacuate meconium plugs during the procedure 1
• Demonstrates the transition zone between narrow aganglionic segment and dilated proximal colon in Hirschsprung disease 1
• Shows microcolon in cases of structural atresia 1

Critical Pitfall to Avoid

The American College of Radiology explicitly advises against assuming Hirschsprung disease and proceeding directly to rectal biopsy, as the clinical presentation is identical for multiple causes of distal obstruction. 1 This is the most common error in this clinical scenario—jumping to biopsy without first performing contrast enema.

Why Other Options Are Incorrect

Plain X-ray (Option A):

• While plain radiographs may show dilated bowel loops and air-fluid levels, they cannot differentiate between the various causes of distal obstruction 2
• Plain films are useful for proximal obstructions but inadequate for distal obstruction diagnosis 1, 2

Rectal Biopsy (Option B):

• Should only be performed AFTER contrast enema confirms findings suggestive of Hirschsprung disease 1
• Biopsy is confirmatory, not the initial diagnostic test 1, 3
• Research shows biopsy correlates well with contrast enema findings, but enema must guide the decision to biopsy 3

Anal Manometry (Option C):

• Also a confirmatory test, not initial diagnostic imaging 3
• Less readily available than contrast enema in most centers 3

US Abdomen (Option D):

• The American College of Radiology explicitly states that ultrasound has no role in evaluating neonates with suspected distal obstruction, as there is no relevant literature supporting its use in this clinical scenario. 1, 2

Algorithmic Approach

1. First: Contrast enema to identify the specific cause of distal obstruction 1
2. If contrast enema shows transition zone: Proceed to rectal biopsy for histologic confirmation of Hirschsprung disease 1, 3
3. If contrast enema shows meconium plug: The enema itself may be therapeutic 1
4. If contrast enema shows microcolon/atresia: Surgical consultation for structural abnormality 1

Given the options provided, if contrast enema is not available as a choice, this represents a flawed question, as rectal biopsy (B) would be premature without contrast enema first. 1