Critical pH — ORE Part 1 MCQ
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Correct answer: B — Approximately 5.5
The correct answer is B (approximately 5.5). This is Stephan's critical pH, the threshold at which salivary pH falls below saturation with respect to enamel mineral (calcium hydroxyapatite), permitting demineralisation to begin. The critical pH of 5.5 is derived from the theoretical solubility chemistry of enamel mineral and is universally taught and examined in dental science, including UK ORE. Distractor B (7.0, neutral pH) is too high—remineralisation occurs above critical pH. C (4.0) and E (3.0) represent severely acidic pH values; while demineralisation is rapid and extensive at these values, they are not the threshold for demineralisation to begin. D (6.5) is incorrect; at this pH enamel is still in saturation equilibrium and remineralisation occurs.
Reference: Microradiography and polarised light imaging study of acid-induced demineralisation (2021, PubMed https://pubmed.ncbi.nlm.nih.gov/32438107/) and peer-reviewed literature on caries risk assessment and Stephan's critical pH model (2020–2025). Consistent with international dental caries prevention science.