MS-2241

A 3-year-old male is brought to the clinician for evaluation of recurrent seizures. He has a history of frequent infections. During chest examination, a heart murmur is detected on the left sternal edge and he presents with abnormal facial features. Laboratory tests show hypocalcemia, hyperphosphatemia, and low levels of parathyroid hormone. The diagnosis is DiGeorge syndrome. Which pharyngeal pouch gives rise to the thymus in this condition?