Embryology USMLE Step 1 Practice Question

A 2-day-old male born at term presents with excessive drooling, respiratory distress with episodes of cyanosis during feeding attempts, and choking. The mother reports polyhydramnios was noted on prenatal ultrasound. Vital signs show HR 148/min, RR 56/min, and SpO2 87% on room air. Physical examination reveals the infant cannot tolerate oral feeds. A chest X-ray shows hyperinflation but no infiltrates. An upper gastrointestinal contrast study demonstrates contrast material unable to progress past the mid-esophagus, with contrast also entering the respiratory tract. Which embryologic process failed to occur properly?