Gross — Esophageal Atresia with Distal TEF Type C

A 2-day-old term male neonate presents with excessive frothy salivation, choking with attempted feeds, and respiratory distress. A nasogastric tube cannot be advanced beyond 10 cm and coils in the upper esophagus on chest X-ray. The abdomen is distended with air-filled stomach visible on imaging. The structure marked **A** in the diagram represents the anatomic defect in this infant. Which of the following is the MOST COMMON embryologic basis for this malformation?