The ability of kidney to form a concentrated urine will decrease if:

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.