<s>[INST] context: The thick ascending limb is permeable to Na+, K+, and Cl- but impermeable to water. Due to this factor, the solutes move out from tubule into the medullary interstitium. NKCC transpoer is responsible for the movement of ions. It is co-transpo in which the other two ions move due to the potential energy stored in the Na+ concentration gradient. Thus, the prime driving force is reabsorption of Na+ This reabsorption, without water being reabsorbed, leads to medullary hyperosmolarity. It is a consequence of countercurrent multiplier system ADH is not the driving force for countercurrent mechanism. Medullary hyperosmolarity can be said to be the driving force for ADH action. Because water can only move passively (driven by an osmotic gradient). Urea does not drive water reabsorption as it is an ineffective osmole. It equilibriates between medullary interstitium and tubular fluid. Thus, it increases hyperosmolarity of interstitium and gets excreted in high concentration in urine. Vasa recta are called "exchangers". They do not actively contribute to the medullary hyperosmolarity, but they prevent it from dissipating. [/INST] question: The prime driving force for counter current multiplier system is: | options: A: Medullary hyperosmolarity B: Reabsorption of Na+ in thick ascending limb C: Action of ADH aquaporin channels D: Urea recycling | answer: B </s>