This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
The ability of kidney to form a concentrated urine will decrease if: 
Select and explain. 
A. The permeability of the collecting duct to water decreases, B. Rate of blood flow through the medulla increases, C. Activity of Na+/K+-pump in the loop of Henle decreases, D. Aquaporin inseion is suppressed.
B: Rate of blood flow through the medulla increases
Answer: B: Rate of blood flow through the medulla increases 
Explanation: Formation of concentrated urine requires creation of a hyperosmolar medullary interstitium. This is achieved by countercurrent multiplier mechanism. The hyperosmolarity of the medullary interstitium is maintained by the vasa recta. The sluggish blood flow in vasa recta ensures that the solute build up in the interstitium is maintained. Increased rate of blood flow through vasa recta will cause solutes to be washed away from the medullary interstitium. In that case, hyperosmolarity of the interstitium will be dissipated, ADH action can not be executed, and the concentrated urine will not form. Options A and D indicate that the action of ADH is suppressed. Urine will be hypotonic with a decreased action of ADH.