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: Medicine
[{'content': 'All of the following substances are added to the urine by means of tubular secretion, except \nSelect and explain. \nA. Potassium, B. Hydrogen ion, C. Urea, D. Creatinine', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]