A 25-year-old woman reports dark spots between her teeth; interproximal caries are found on teeth 7, 8, 9 and 10. A plaque pH tracing following a single acid challenge is shown. How long does it take for the plaque pH to drop to the critical pH and then to neutralize?

Correct answer: D. 5 minutes; 40 minutes

• A. 10 minutes overstates the fall. The steep descent has already bottomed out at around 5 minutes, and the curve is flat at its minimum between roughly 5 and 8 minutes.
• B. The 5-minute fall is right but 20 minutes is too fast for full recovery. The tracing is still climbing at 20 minutes and does not flatten at baseline until it approaches the 40-minute mark.
• C. 15 minutes is well past the minimum — by then the pH is already recovering. The 40-minute recovery figure is correct, but the fall time is wrong.
• D. CORRECT, and this is the standard Stephan curve. The fall is rapid: bacterial fermentation drives plaque pH from neutral down through the critical threshold to its minimum within about 5 minutes. The recovery is slow and asymmetric: salivary buffering, clearance and remineralization return the pH to baseline only after roughly 40 minutes. That asymmetry — fast down, slow up — is the whole clinical point.

Note: The Stephan curve explains why snacking frequency dominates caries risk: each exposure costs roughly 40 minutes below baseline, so six snacks a day means about four hours of net demineralization, while the same sugar taken at one sitting costs 40 minutes. It also explains why the interproximal surfaces in this patient are affected — plaque is thickest and salivary clearance poorest there.