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: Pharmacology
[{'content': 'The primary mechanism of action of fluoride on topical application is - \nSelect and explain. \nA. Conversion of hydroxyapatite to fluoroapatite by replacing the OH- ions, B. Inhibition of plaque bacteria, C. Form a reservoir in saliva, D. Improvement in tooth morphology', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Conversion of hydroxy-apatite to fluro-apatite by replacing the OH ions When hydroxyapetite is exposed to low fluoride concentrations (about 1 ppm) a layer offluorapetite forms on the hydroxyapetite crystals. The thin layer governs the rate of enamel dissolutin and reduces rate of caries progression. o During caries attack, the enamel is exposed to a pH of about 5.5 or lower, thereby leading to its dissolution. o The presence of fluoride reduces the solubility of enamel by promoting the precipitation of hydroxyapetite and phosphate mineral.", 'role': 'assistant'}]