A 13-year-old male attends with his parent, who says: "My son's teeth are bothering him even though he eats the same amount as his older sister, who has no problems." Multiple new carious and incipient lesions are found. Two 12-hour plaque pH tracings are shown. Which graph most likely belongs to this patient?

Correct answer: B. Graph #2, because there is a higher frequency of critical acid attack

• A. The observation is broadly true of Graph #2 but the reasoning is back-to-front. Reduced neutral time is the CONSEQUENCE of frequent acid challenges, not an independent cause. The examiner wants the cause — higher frequency — named directly.
• B. CORRECT. The parent's remark is the key to the whole item: the two children eat the SAME AMOUNT, so total sugar quantity cannot explain the difference. What differs is FREQUENCY. Graph #2 shows many more excursions below the critical pH line than Graph #1, meaning the enamel spends far more separate episodes in a demineralizing state with less recovery time between them. Frequency of acid challenge, not the total amount of sugar, is the dominant dietary determinant of caries risk.
• C. This misreads the tracing and also misstates the mechanism. Acid does not 'accumulate' at particular time points; each challenge is cleared by salivary buffering, and the injury comes from how frequently that clearance is interrupted.
• D. Graph #1 does contain the single deepest drops, but depth is not the discriminator here. A few very acidic excursions with long neutral recovery periods in between are less cariogenic than many shallower ones. Depth of the pH fall matters far less than how often it happens.

Note: The classic teaching point, and a favourite INBDE stem: it is FREQUENCY of fermentable carbohydrate exposure, not total quantity, that drives caries risk. A can of soda drunk in one sitting produces one Stephan curve; the same can sipped over three hours produces many. Counsel patients on how often they eat, not just what.