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': 'Urinary concentrating ability of the kidney is increased by \nSelect and explain. \nA. ECF volume contraction, B. Increase in renal blood flow, C. Reduction of medullary hyperosmolarity, D. Increase in GFR', 'role': 'user'}, {'content': 'Answer: A \nExplanation: When there is an ECF volume expansion, there is increased renal blood flow and increased GFR. When renal blood flow is more, the solutes in renal medulla are washed off. And this reduces the medullary concentration. Similarly, when increased GFR, Na+ reabsorption power is decreased, this leads to a decrease of the medullary concentration. When ECF volume contraction occurs, the medullary concentration is increased.', 'role': 'assistant'}]