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
[{'content': 'Which of the following is not a component of countercurrent mechanism? \nSelect and explain. \nA. Thick ascending limb of henle, B. Collecting duct, C. Descending limb of henle, D. Vasa recta', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The concentrating mechanism depends upon 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. A countercurrent system is a system in which the inflow runs parallel to, counter to, and in close proximity to the outflow for some distance. This occurs for both the loops of Henle and the vasa recta in the renal medullaRef: Ganong&;s Review of Medical Physiology, 23rd edition, page no: 654', 'role': 'assistant'}]