What is the approximate critical pH below which ROOT surface (dentin/cementum) demineralisation begins?

Correct answer: A. 6.5

• A. Correct. Root surface dentin and cementum demineralise at a HIGHER pH than enamel — reported in the literature between roughly 6.2 and 6.7, commonly quoted as about 6.5. Because the threshold is higher, root surfaces begin dissolving at a pH that would leave enamel entirely unaffected. This explains why exposed root surfaces in older adults are so vulnerable: even mildly acidic challenges that pose no threat to enamel will demineralise cementum and dentin.
• B. 4.5 is well below the threshold for any dental hard tissue and is typical of soft drinks.
• C. 7.4 is physiological blood pH — no demineralisation occurs at neutrality.
• D. 5.5 is the classic critical pH for ENAMEL, not for root surface.