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
Urine concentrating ability of the kidney is increased by 
Select and explain. 
A. Increased medullary hyperosmolarity, B. Increase in RBF, C. Reduction of medullary hyperosmolarity, D. Increase in GFR
A: Increased medullary hyperosmolarity
Answer: A: Increased medullary hyperosmolarity 
Explanation: 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