<prompt> The primary driving force for counter current Multiplier system is:2018Medullary hyperosmolarityReabsorption of Na in thick ascending limbAction of ADH aquaporin channelsUrea recycling  <response>: Concentrating mechanism depends on the maintenance of a gradient of increasing osmolality along the medullary pyramids. This gradient is produced by the operation of the loops of Henle as countercurrent multipliers and maintained by the operation of the vasa recta as countercurrent exchangers. Ref:- Ganong, pg num:- 685 