A 28-week preterm infant develops respiratory distress within 2 hours of birth. Chest X-ray shows a ground-glass appearance with air bronchograms. A 32-week preterm infant born to a mother who received antenatal corticosteroids presents with tachypnea and grunting at 6 hours of life, with hyperinflation on CXR. Which single feature best distinguishes Respiratory Distress Syndrome (RDS) from Transient Tachypnea of the Newborn (TTN)?