A 26-week preterm infant is born to a mother with preeclampsia who did not receive antenatal steroids. At 4 hours of life, the infant develops severe respiratory distress with grunting, nasal flaring, and intercostal retractions. ABG shows pH 7.20, PaCO₂ 65 mmHg, PaO₂ 45 mmHg on FiO₂ 0.80. Surfactant is administered. A term infant born to a diabetic mother presents with tachypnea (RR 65/min) and mild retractions at 2 hours of life; ABG is normal. CXR shows hyperinflation without ground-glass appearance. Which feature most reliably distinguishes RDS from Transient Tachypnea of the Newborn in this scenario?