This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Vasopressin acts by: \nSelect and explain. \nA. Water transpo across collecting duct, B. Water absorption at medullary ducts, C. Water secretion at loop of Henle, D. Water transpo at PCT', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Water transpo across collecting ductVasopressin acts by increasing permeability of collecting ductsQ. to water. The key to the action of vasopressin on collecting duct is aquaporin -2 . The overall effect is retention of water in excess of solute which results in decrease effective osmotic pressure', 'role': 'assistant'}]