A 21-day-old boy is brought to the office by his mother because of a palpable bulge in the child’s neck.  He continues to feed well but appears comfortable only when held with his body sideways under the breast.  He is at the 50th percentile for height, weight, and head circumference.  The child favors looking toward the right and cries when his head is turned to the left.  There is a firm mass on the left side of his neck that does not move when the child swallows.  The remainder of the examination is unremarkable.  Which of the following conditions was most likely present prenatally?