A 32-week-old male neonate born to a mother who did not receive antenatal corticosteroids presents with severe respiratory distress within 2 hours of birth. CXR shows a ground-glass reticulonodular pattern with air bronchograms and decreased lung volume. Amniotic fluid analysis reveals the parameter marked **D** in the diagram is 1.5. Which of the following best explains the pathophysiology of this infant's condition?

Explanation

## Why "Loss of surface tension reduction in alveoli leads to atelectasis and decreased lung compliance" is right The parameter marked **D** (L:S ratio > 2.0) indicates fetal lung maturity, typically achieved at ~35 weeks gestation. This infant's L:S ratio of 1.5 is below the threshold for maturity, indicating surfactant deficiency. Surfactant, primarily dipalmitoylphosphatidylcholine (DPPC), is produced by type II pneumocytes but adequate amounts are not present until ~35 weeks. Without sufficient surfactant, the alveoli cannot reduce surface tension on expiration, leading to alveolar collapse (atelectasis), decreased compliance, and the characteristic ground-glass appearance on CXR with air bronchograms. This is neonatal respiratory distress syndrome (RDS, hyaline membrane disease). [Nelson 21e Ch 122] ## Why each distractor is wrong - **Retained fetal lung fluid in the terminal airways causes self-limited tachypnea resolving within 48 hours**: This describes transient tachypnea of the newborn (TTN), which is distinct from RDS. TTN is typically seen after cesarean delivery without labor and resolves spontaneously. This infant has RDS with a pathologic CXR pattern and severe distress, not TTN. - **Meconium aspiration into distal airways creates a ball-valve obstruction with air trapping**: Meconium aspiration syndrome (MAS) presents with hyperinflation and air trapping on CXR, not the ground-glass pattern with decreased lung volume seen here. There is no mention of meconium-stained amniotic fluid. - **Inadequate closure of the foramen ovale results in right-to-left shunting and hypoxemia**: While hypoxemia is present in RDS, it is due to atelectasis and ventilation-perfusion mismatch from surfactant deficiency, not from a patent foramen ovale. PFO closure is a normal postnatal event unrelated to surfactant status. **High-Yield:** L:S ratio > 2.0 = fetal lung maturity at ~35 weeks; ratio < 2.0 = surfactant deficiency → RDS in preterm infants. [cite: Nelson 21e Ch 122]