This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
The region of a nephron where P cells (principal cells) are located is: 
Select and explain. 
A. PCT, B. Loop of Henle, C. DCT, D. Collecting duct
D: Collecting duct
Answer: D: Collecting duct 
Explanation: Collecting ducts have 2 cell populations: P cell / principle cell: Reabsorb Na+Secrete K+ Intercalated cell: Absorb K+Secrete H+, HCO3- it is the buffering cell in the kidney.