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': 'Which of the following is the primary mechanism of action of fluoride on topical application? \nSelect and explain. \nA. Forming a reservoir in saliva, B. Inhibition of plaque bacteria, C. Improvement in tooth morphology, D. Conversion of hydroxyapatite to fluorapatite by replacing the OH- ions', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The presence of fluoride reduces the solubility of enamel by promoting the precipitation of hydroxyapatite and phosphate mineral. Ca10(P04)6(OH)2 +2F- -> Ca10(PO4)6F2 + 2OH (OH ions are replaced) (hydroxy-apatite) (fluro-apatite) Ref: Essentials of Pedodontics By Tilakraj, Pages 182-3 ; A Textbook of Public Health Dentistry By Marya, Page 348', 'role': 'assistant'}]