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 buffer with the highest pKa is: 
Select and explain. 
A. Phosphate, B. Bicarbonate, C. Intracellular proteins, D. Ammonia
D: Ammonia
Answer: D: Ammonia 
Explanation: Buffer PKa Phosphate 6.8 Bicarbonate 6.1 Intracellular proteins 7.2 Ammonia 9.0 The only basic buffer is ammonia.