<prompt> Urine concentrating ability of the kidney is increased byIncreased medullary hyperosmolarityIncrease in RBFReduction of medullary hyperosmolarityIncrease in GFR  <response>: 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 