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: Dental
[{'content': 'The mechanism of action of APF is via following reaction products: \nSelect and explain. \nA. Fluoridated hydroxyapatite, B. Stannous trifluorophosphate, C. Dicalcium phosphate dihydrate (DCPD), D. DCPD + Calcium fluoride + fluorapatite', 'role': 'user'}, {'content': 'Answer: D \nExplanation: When APF is applied on the teeth, it initially leads to dehydration and shrinkage in the volume of hydroxypatite crystals which further on hydrolysis forms an intermediate product called dicalcium phosphate dihydrate (DCPD). This DCPD is highly reactive with fluoride ion and starts forming immediately when APF is applied.\nFluoride penetrates into the crystals more deeply through the openings produced by shrinkage and leads to formation of fluorapatite.\nFor the conversion of whole of DCPD so formed into fluorapatite, deeper penetration and continuous supply of fluoride is required. Because of this reason, APF is applied every\xa0 30 seconds and the teeth has to be kept wet for 4 minutes.\xa0\nBecause high fluoride concentration and low pH favor fluoride deposition, acidification of the fluoride solution with phosphoric acid was found to suppress the dissolution of enamel, as well as the formation of calcium fluoride and provide a more effective treatment.\xa0\nThe intermediate product formed is the dicalcium phosphate dihydrate and calcium fluoride is the principal reaction product.\n\n\u200bEssentials of preventive and community dentistry\xa0\nSoben Peter\xa0\n5th edition', 'role': 'assistant'}]