A Stephan curve is shown with four labelled bands (A, B, C, D) defined by the salivary pH trace, the enamel critical pH, the fluorapatite critical pH and the dentin critical pH. Which section(s) represent demineralization of HYDROXYAPATITE following the acid challenge?

Correct answer: C. C + D

• A. Band B lies between the dentin critical pH (6.2, dashed) and the enamel critical pH (5.5, solid). In this window DENTIN and cementum dissolve but enamel does not — hydroxyapatite is still stable. This is the band that explains why root caries begins at a higher pH than enamel caries.
• B. This includes band B, where enamel is not yet dissolving, and omits band D, where it dissolves fastest. It is wrong at both ends.
• C. CORRECT. Hydroxyapatite is the mineral of ENAMEL, and its critical pH is 5.5 — the solid line on the graph. Demineralization of hydroxyapatite occurs wherever the trace falls BELOW that line, which is every region beneath it: band C (between 5.5 and the fluorapatite line at 4.5) and band D (below 4.5). Both must be included.
• D. Band C alone is incomplete. It covers pH 4.5 to 5.5, but the trace continues below 4.5 into band D, where hydroxyapatite dissolution not only continues but accelerates. Stopping at C omits the most aggressive part of the challenge.

Note: Map the three thresholds onto the bands: above 6.2 (band A) nothing demineralizes; 5.5–6.2 (band B) dentin and cementum only; 4.5–5.5 (band C) enamel hydroxyapatite as well; below 4.5 (band D) even fluorapatite dissolves. This is exactly why fluoride works — converting hydroxyapatite to fluorapatite lowers the critical pH from 5.5 to 4.5, so band C becomes protective rather than destructive.