What is the mechanism of action of fluoride?

Correct answer: D. Reduces the solubility of calcium phosphate (forms fluorapatite)

• A. Fluoride facilitates the deposition of fluorapatite (not hydroxyapatite) during remineralization.
• B. Fluoride does not reduce hydroxyapatite concentration — it CONVERTS hydroxyapatite to fluorapatite, which is stronger and more acid-resistant.
• C. At low concentrations (fluoride in toothpaste/water), fluoride has some antimicrobial effect (inhibits bacterial enolase) but does NOT kill all oral bacteria. This is not its primary mechanism.
• D. Fluoride's primary cariostatic mechanism is the formation of FLUORAPATITE (by substituting hydroxyl groups in hydroxyapatite), which is significantly LESS SOLUBLE in acid than hydroxyapatite. This means fluoride-rich enamel is MORE RESISTANT to acid dissolution at lower pH values (the critical pH for fluorapatite is ~4.5 vs. 5.5 for hydroxyapatite). Additional mechanisms: promotes remineralization, inhibits bacterial glycolysis.