Embryology USMLE Step 1 Practice Question

A 6-week-old male is brought to the pediatric clinic by his mother, who noticed a soft, painless mass on the right side of his neck present since birth. Vital signs are normal. On physical examination, the mass is located anterior to the sternocleidomastoid muscle at the angle of the mandible. It transilluminates brightly, does not move with swallowing or tongue protrusion, and becomes more prominent when the infant cries. Ultrasound shows an 8 × 6 mm anechoic fluid-filled lesion with connections to lymphoid tissue near the tonsillar fossa. There is no overlying skin dimple, fistula, or discharge. Which embryologic structure is responsible for this finding?