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
[{'content': 'The buffer with the highest pKa is: \nSelect and explain. \nA. Phosphate, B. Bicarbonate, C. Intracellular proteins, D. Ammonia', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Buffer PKa Phosphate 6.8 Bicarbonate 6.1 Intracellular proteins 7.2 Ammonia 9.0 The only basic buffer is ammonia.', 'role': 'assistant'}]