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': 'Substance involved in countercurrent mechanism for maintaining medullary gradient - \nSelect and explain. \nA. NaCl, B. Urea, C. NaCl, urea, D. NaCl, urea, water', 'role': 'user'}, {'content': 'Answer: D \nExplanation: D i.e. NaCI, Urea, Water', 'role': 'assistant'}]