This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Renal physiology, Subject name: Physiology
The prime driving force for countercurrent multiplier system is 
Select and explain. 
A. Medullary hyperosmolarity, B. Reabsorption of Na+ in thick ascending limb, C. Action of ADH aquaporin channels, D. Urea recycling
B: Reabsorption of Na+ in thick ascending limb
Answer: B: Reabsorption of Na+ in thick ascending limb 
Explanation: In juxtamedullary nephrons with longer loops and thin ascending limbs, the osmotic gradient is spread over a greater distance and the osmolality at the tip of the loop is greater. This is because the thin ascending limb is relatively impermeable to water but permeable to Na+ and Cl-. Therefore, Na+ and Cl- move down their concentration gradients into the interstitium, and there is additional passive countercurrent multiplication.Ref: Ganong&;s Review of Medical Physiology, 23rd edition, page no: 655