A 2-day-old newborn male is cyanotic after attempts to swallow milk result in collection of the milk in his mouth. After 2 days he develops pneumonia. A tracheoesophageal fistula is suspected. Which of the following structures has failed to develop properly?

The tracheoesophageal septum is the down-growth that separates the ventral wall of the foregut (esophagus) from the laryngotracheal tube. The presence of a fistula would result in passage of fluid from the esophagus into the trachea and could cause pneumonia. If the esophagus did not develop correctly, as in esophageal atresia, it would end as a blind tube. This kind of defect, although associated with tracheoesophageal fistula, is not the result of an opening into the trachea, and pneumonia would not result. Abnormal tracheal development can be associated with tracheoesophageal fistula, therefore, but it is not the direct cause of it. Abnormal tongue development does not result in a tracheoesophageal fistula. Abnormal development of the pharynx is not associated with a tracheoesophageal fistula.