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
Which of the following is the mode of action of vasopressin? 
Select and explain. 
A. Facilitates water absorption at medullary ducts, B. Increasing permeability of H2O across collecting duct, C. Facilitate water absorption at PCT, D. Facilitate H2O secretion at loop of henle
B: Increasing permeability of H2O across collecting duct
Answer: B: Increasing permeability of H2O across collecting duct 
Explanation: The action of vasopressin is increasing the permeability of collecting duct to water Aquaporin 2. The net effect is water retention in excess solute resulting in decreased effective osmotic pressure of body fluids. Ref: Ganong, 21st Edition, Page 235