This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Renal physiology, Subject name: Physiology
[{'content': 'Urine concentrating ability of the kidney is increased by \nSelect and explain. \nA. Increased medullary hyperosmolarity, B. Increase in RBF, C. Reduction of medullary hyperosmolarity, D. Increase in GFR', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Countercurrent mechanism is a system in the renal medulla that facilitates concentration of the urine as it passes through the renal tubules.Ref: Ganong&;s review of medical physiology; 24th edition; page no: 687', 'role': 'assistant'}]