<prompt> Evaluation of an infant reveals hypocalcemia due to a lack of parathyroid hormone. On x-ray, the thymic shadow is absent. Which of the following embryonic structures is involvedThird pharyngeal cleftFouh pharyngeal cleftThird pharyngeal pouchFouh pharyngeal pouch  <response>: Since thymus is missing, appropriate answer would be third pharyngeal pouch. This patient has got DiGeorge syndrome. DiGeorge syndrome causes migration defects of neural crest-derived tissues, paicularly affecting development of the third and fouh Branchial pouch pharyngeal pouches 