A 32-week preterm male infant born to a mother with poorly controlled gestational diabetes presents with respiratory distress at 4 hours of life. Which radiologic finding best distinguishes Meconium Aspiration Syndrome (MAS) from Respiratory Distress Syndrome (RDS)?

## Radiologic Discrimination: MAS vs. RDS ### Chest X-Ray Patterns **Key Point:** MAS presents with a characteristic **hyperinflated** chest with **patchy, irregular infiltrates and areas of atelectasis** (the "ball-valve" obstruction pattern). RDS shows **uniform ground-glass opacity** with **air bronchograms** reflecting diffuse alveolar collapse. | Feature | MAS | RDS | |---------|-----|-----| | **Lung volumes** | Hyperinflated (air trapping) | Normal or reduced | | **Infiltrate pattern** | Patchy, irregular, asymmetric | Uniform, diffuse, symmetric | | **Air bronchograms** | Absent or minimal | Prominent (classic finding) | | **Atelectasis** | Focal areas (from obstruction) | Diffuse (from collapse) | | **Pneumothorax/pneumomediastinum** | Common (barotrauma) | Rare | | **Mechanism** | Meconium plugging → ball-valve obstruction | Surfactant deficiency → alveolar collapse | ### Pathophysiologic Basis **High-Yield:** The radiologic differences reflect the underlying mechanisms: 1. **MAS:** Meconium particles lodge in airways, causing **ball-valve obstruction** → air enters distal alveoli during inspiration but cannot exit during expiration → hyperinflation and patchy infiltrates (meconium + inflammation). 2. **RDS:** Surfactant deficiency → alveolar collapse throughout the lung → uniform ground-glass appearance (collapsed alveoli + pulmonary edema fluid). **Clinical Pearl:** MAS is more common in **term and post-term infants** (meconium passage is a sign of fetal maturity). RDS is most common in **preterm infants <34 weeks**. The clinical context (gestational age, presence of meconium-stained amniotic fluid) combined with the chest X-ray pattern allows reliable differentiation. ### Why Other Options Are Not Discriminators - **Bilateral perihilar infiltrates with fluid in fissures (Option 2):** This pattern is more typical of TTN or neonatal pneumonia, not MAS or RDS. - **Unilateral lobar consolidation (Option 3):** This suggests focal pneumonia or aspiration of gastric contents, not the diffuse or patchy patterns of MAS or RDS. **Mnemonic:** **MAS-HI** = **M**econium **A**spiration **S**yndrome → **H**yperinflation with **I**rregular infiltrates; **RDS-GG** = **R**espiratory **D**istress **S**yndrome → **G**round-**G**lass appearance.