Fluoride's mechanisms of action include all of the following EXCEPT:

Correct answer: A. Increases the critical pH value

• A. CORRECT (EXCEPTION) — Fluoride LOWERS the critical pH (the pH at which enamel begins to dissolve). Fluorapatite is more resistant to dissolution and has a lower critical pH (approximately 4.5 vs. 5.5 for hydroxyapatite). Increasing the critical pH would mean enamel dissolves at a higher pH — which is incorrect for fluoride's effect.
• B. INCORRECT — Remineralization is a primary mechanism of fluoride; fluoride ions promote remineralization of early carious lesions by incorporating into the hydroxyapatite crystal lattice as fluorapatite.
• C. INCORRECT — Fluoride interferes with bacterial glycolysis and inhibits enolase in cariogenic bacteria (especially S. mutans), reducing acid production — this IS a correct mechanism.
• D. INCORRECT — Fluoride decreases enamel solubility by forming fluorapatite, which is more resistant to acid dissolution than hydroxyapatite — this IS a correct mechanism.