Meconium Aspiration Syndrome
The most likely diagnosis is C. Meconium aspiration syndrome (MAS), based on the classic triad of post-term delivery, meconium-stained amniotic fluid with fetal distress, and the pathognomonic chest X-ray findings of hyperinflated lungs with patchy infiltrates and areas of atelectasis. 1
Diagnostic Reasoning
Why MAS is the Correct Answer
This clinical presentation is textbook for meconium aspiration syndrome. The key diagnostic features that definitively point to MAS include:
- Post-term status (≥42 weeks) significantly increases the risk of meconium-stained amniotic fluid, occurring in 5-15% of deliveries, and post-term infants are at highest risk 1, 2
- History of fetal distress with meconium-stained amniotic fluid is the classic antecedent event, with approximately 3-5% of neonates born through meconium-stained fluid developing MAS 1, 2
- The chest X-ray findings are pathognomonic: hyperinflated lungs with patchy infiltrates and areas of atelectasis are the hallmark radiographic features of MAS 3, 4
Why the Other Options Are Incorrect
Transient tachypnea of the newborn (Option A) is excluded because TTN typically presents with perihilar streaking and fluid in the fissures on chest X-ray, not hyperinflation with patchy infiltrates 1. TTN also lacks the history of meconium-stained fluid and fetal distress.
Respiratory distress syndrome (Option B) is definitively ruled out because RDS occurs primarily in premature infants (<1,500 g birth weight), not post-term infants, and shows a characteristic ground-glass appearance with air bronchograms on imaging—completely different from this patient's radiographic findings 1.
Neonatal pneumonia (Option D) is less likely because while pneumonia can present with respiratory distress and infiltrates, it would not typically produce the characteristic hyperinflation pattern seen here, nor would it have the specific context of post-term delivery with meconium-stained fluid and fetal distress 5.
Understanding the Pathophysiology
The clinical picture reflects the multifactorial pathophysiology of MAS:
- Mechanical airway obstruction from aspirated meconium causes the hyperinflation (ball-valve effect) 6, 4
- Chemical pneumonitis from meconium creates the patchy infiltrates 6, 4
- Surfactant inactivation by meconium contributes to areas of atelectasis 5, 6
- Pulmonary inflammation and potential persistent pulmonary hypertension complicate the clinical course 6, 4
Critical Clinical Pearls
The diagnosis of MAS is essentially clinical: respiratory distress in a newborn delivered through meconium-stained amniotic fluid whose symptoms cannot otherwise be explained 6, 3, 7. The radiographic findings confirm what the clinical history already strongly suggests.
Post-term status is a major risk factor because meconium passage is related to gastrointestinal tract maturation and typically occurs near or post-term 6. The combination of post-term delivery, fetal distress, and meconium-stained fluid creates the perfect storm for MAS development.
Thick meconium carries the highest risk: infants with thick meconium-stained amniotic fluid have a 61% incidence of developing MAS, compared to only 0.3% with light meconium 8. Male infants and those who are small-for-gestational-age face additional risk 8.