A male infant born at 28 weeks gestation (birth weight 1100 g) presents with grunting, nasal flaring, intercostal retractions, tachypnea (RR 80), and central cyanosis at 30 minutes of life. SpO₂ is 75% on room air. Chest radiograph shows bilateral, diffusely low lung volumes with fine reticulogranular ('ground glass') opacities throughout both lungs and prominent air bronchograms extending peripherally. The condition marked **A** in the diagram is suspected. Which of the following best describes the PRIMARY PATHOPHYSIOLOGIC MECHANISM underlying this condition?

Explanation

## Why option 1 is correct The clinical presentation, radiographic findings (bilateral low volumes, reticulogranular opacities, air bronchograms), and gestational age (28 weeks) are pathognomonic for hyaline membrane disease (neonatal RDS). The PRIMARY PATHOPHYSIOLOGIC MECHANISM is **deficiency of pulmonary surfactant**, a lipoprotein complex (90% lipid, predominantly dipalmitoyl-phosphatidylcholine/lecithin, and 10% protein including surfactant proteins SP-A, SP-B, SP-C, SP-D) produced by type II pneumocytes. Surfactant lowers alveolar surface tension according to the Laplace law (P = 2T/r), preventing end-expiratory alveolar collapse and stabilizing alveoli of different sizes. At 28 weeks gestation, surfactant production is inadequate (adequate quantities only by 34–36 weeks), resulting in diffuse atelectasis, decreased compliance, ventilation-perfusion mismatch, and the characteristic hyaline membrane formation (eosinophilic protein-rich exudate lining alveoli). This is the textbook anchor from Nelson Pediatrics 22e and AAP/ACOG guidelines. ## Why each distractor is wrong - **Option 2 (Transient tachypnea of newborn)**: TTN is caused by delayed clearance of fetal lung fluid and presents with wet lungs, pleural effusions, and hyperinflation (not low volumes or ground-glass opacities). TTN typically occurs in term or near-term infants and is self-limited. This is the pathophysiology of structure **B**, not **A**. - **Option 3 (Meconium aspiration syndrome)**: MAS results from aspiration of meconium-stained amniotic fluid, causing chemical pneumonitis and airway obstruction. It typically presents in term infants with perinatal asphyxia and shows patchy infiltrates with hyperinflation, not the diffuse fine reticulogranularity and low volumes seen here. This is the pathophysiology of structure **C**, not **A**. - **Option 4 (Congenital pneumonia)**: Congenital pneumonia results from bacterial colonization in utero or during delivery, presenting with inflammatory infiltration. It may mimic RDS radiographically but the primary mechanism is infection, not surfactant deficiency. The clinical context (no maternal chorioamnionitis, no fever) and classic radiographic pattern favor HMD. This is the pathophysiology of structure **D**, not **A**. **High-Yield:** Hyaline membrane disease = surfactant deficiency in premature infants; antenatal corticosteroids (betamethasone 12 mg IM × 2 doses 24 h apart) accelerate surfactant production and reduce RDS incidence by ~50% (Liggins trial). [cite: Nelson Pediatrics 22e — Neonatal RDS; AAP/ACOG Antenatal Steroids]