This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
Which of the pharyngeal pouch contributes to the thymus? 
Select and explain. 
A. The second pharyngeal pouch, B. The first pharyngeal pouch, C. The third pharyngeal pouch, D. The fifth pharyngeal pouch
C: The third pharyngeal pouch
Answer: C: The third pharyngeal pouch 
Explanation: The elongate ventral poions of the third pair of pharyngeal pouches migrate medially and fuse to form the primordium of the thymus gland. Development of the thymus is not complete at bih; it contains to grow and reaches its greatest size at pubey, after which it gradually shrinks and its functional tissue is replaced by a fatty tissue. Ref: Ochs H.D., Notarangelo L.D. (2010). Chapter 82. Immunodeficiency Diseases. In J.T. Prchal, K. Kaushansky, M.A. Lichtman, T.J. Kipps, U. Seligsohn (Eds),Williams Hematology, 8e.