In DiGeorge syndrome, thymic aplasia predisposes to T-cell deficiency. The thymus and parathyroid glands originate from which embryologic structures?

• A. Third and fourth pharyngeal pouches
• B. First pharyngeal pouch
• C. Neural crest cells
• D. Endodermal outpouchings

Answer: (A)

Thymus and parathyroid glands develop from the pharyngeal pouches during embryogenesis. The thymus and inferior parathyroids come from the third pharyngeal pouch, while the superior parathyroids come from the fourth pouch. In DiGeorge syndrome, defective development of these pouches—especially the third and fourth—leads to thymic aplasia with T-cell deficiency and hypoparathyroidism. So, the structures in question share their origin in the third and fourth pharyngeal pouches.