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
[{'content': 'Limiting pH in humans is: \nSelect and explain. \nA. 5.5, B. 5, C. 4.5, D. 4', 'role': 'user'}, {'content': "Answer: C \nExplanation: The amount of acid secreted depends upon the subsequent events in the tubular urine. The maximal H+ gradient against which the transpo mechanisms can secrete in humans corresponds to a urine pH of about 4.5; that is, an H+ concentration in the urine that is 1000 times the concentration in plasma. pH 4.5 is thus the limiting pH. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 40.", 'role': 'assistant'}]