A patient is drinking soda and has many caries. Below what approximate pH does enamel (hydroxyapatite) begin to demineralize (the 'critical pH')?

Correct answer: C. 5.5

• A. 7.0 is neutral pH; enamel is stable here.
• B. 2.5 is roughly the pH of the soda itself (very acidic), not the critical demineralization threshold of enamel.
• C. Correct. The critical pH for enamel (hydroxyapatite) demineralization is about 5.5. Below this, the oral fluid becomes undersaturated with respect to hydroxyapatite and enamel begins to dissolve. (Fluorapatite is more resistant, with a lower critical pH near 4.5.)

Note: CORRECTION: the source wrote '5,' but the standard critical pH for enamel/hydroxyapatite demineralization is 5.5 (≈4.5 for fluorapatite). Keyed to 5.5.