Which best describes the initiating step in the caries process at the level of bacterial metabolism?

Correct answer: D. Fermentable carbohydrates are metabolized by biofilm bacteria to organic acids, lowering plaque pH below the critical value

• A. Chelation without pH change is not the accepted mechanism; acid production and the resulting pH drop are the established initiating step.
• B. Protein digestion is not the initiating event. Enamel is roughly 96% mineral by weight, and the proteolytic phase, if it occurs, follows mineral dissolution.
• C. Salivary amylase hydrolyzes starch into fermentable sugars — it supplies substrate for the bacteria, but it does not dissolve enamel.
• D. CORRECT. Cariogenic bacteria — principally mutans streptococci and lactobacilli — ferment dietary carbohydrates (especially SUCROSE) via glycolysis, producing ORGANIC ACIDS, chiefly LACTIC acid. These acids drop the pH within the biofilm below the CRITICAL pH (approximately 5.5 for enamel, 6.2 for root dentin), at which point hydroxyapatite becomes soluble and calcium and phosphate diffuse out of the tooth. Demineralization and remineralization alternate continuously; caries results when the balance tips toward net mineral loss.

Note: Sucrose is uniquely cariogenic because it is BOTH readily fermented to acid AND the substrate from which glucosyltransferase builds extracellular glucans, the sticky scaffold of the biofilm. The Stephan curve — the pH drop and its slow recovery after a sugar exposure — is why FREQUENCY of intake matters more than total quantity.