A smoker who lives on breath mints

Correct answer: B. The frequent sugar-containing mints

• A. Smoking is a dominant risk factor for periodontal disease and oral cancer, and it contributes here to the staining and to reduced salivary flow — but it is not the primary driver of interproximal caries. It is the confounder the stem places in front of you.
• B. CORRECT. Caries risk is governed by the FREQUENCY of fermentable carbohydrate exposure far more than by the total quantity consumed. Each mint drops plaque pH below the critical value — about 5.5 for enamel and 6.2 for root dentine — and it takes 20 to 40 minutes for saliva to buffer it back. At fifteen to twenty exposures spread across the working day, the mouth spends most of its waking hours below the critical pH and is never given a window in which to remineralise. That is the Stephan curve applied to a real habit, and it explains a generalised pattern in the stagnation areas better than anything else in the history.
• C. Brushing certainly matters, but he brushes twice daily with fluoride toothpaste — and a toothbrush does not clean interproximal surfaces at all, whatever the technique. The relevant hygiene gap is INTERDENTAL cleaning, and even correcting it would not undo twenty acid challenges a day.
• D. Unsweetened coffee is mildly acidic and can contribute to erosion over time, but with no sugar it supplies no substrate for acidogenic bacteria and is not cariogenic in the way the mints are.

Note: The counselling that follows is specific rather than general: substitute SUGAR-FREE mints or xylitol products, which do not lower plaque pH and, in the case of xylitol, are not fermentable by Streptococcus mutans. Add interdental cleaning, high-fluoride dentifrice at 5,000 ppm, and address the xerostomia that the smoking is aggravating.

Correct answer: C. Its substantivity

• A. Arrested lesions do become more mineralised and can read as more opaque radiographically, but that is a CONSEQUENCE of the arrest rather than the property that produces it.
• B. Thixotropy describes a gel that thins under shear and stiffens at rest — a handling characteristic of certain fluoride gels. It has nothing to do with duration of action.
• C. CORRECT. SUBSTANTIVITY is the capacity of an agent to bind to a surface and then be released slowly, so that its action persists long after the application itself has ended. SDF deposits silver phosphate and calcium fluoride reservoirs within the carious lesion; the silver binds bacterial proteins and remains bound to the arrested surface, while fluoride is released over time to drive remineralisation and inhibit collagen breakdown. That is why a single application arrests a lesion and why reapplication is scheduled at roughly six-monthly intervals rather than daily. CHLORHEXIDINE is the other classic substantive agent, which is why it retains activity in the mouth for hours after rinsing.
• D. SDF is well tolerated, but biocompatibility describes the absence of an adverse tissue response — not sustained antimicrobial action. Every material in routine use is biocompatible; only some are substantive.

Note: The trade-off is the BLACK STAINING of the arrested lesion, which is permanent on that tissue and must be consented to explicitly before application — it is the single commonest source of complaint. Avoid SDF in patients with a silver allergy and on ulcerated or actively inflamed mucosa.