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: Physiology
Number of aquaporins cloned is: 
Select and explain. 
A. 5, B. 8, C. 13, D. None of the above
C: 13
Answer: C: 13 
Explanation: Rapid diffusion of water across cell membranes depends on the presence of water channels, integral membrane proteins called aquaporins. To date, 13 aquaporins have been cloned; however, only 4 aquaporins (aquaporin-1, aquaporin-2, aquaporin-3, and aquaporin-4) play a key role in the kidney. The roles played by aquaporin-1 and aquaporin-2 in renal water transpo are discussed below. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 38.