A 56-year-old man reports cavities and a dry mouth. He has a history of mucoepidermoid carcinoma of the parotid gland treated with radiotherapy to the jaw. He has several carious lesions and xerostomia. His 12-hour salivary pH tracing is shown. Where is the most likely site of the decay?

Correct answer: A. Root surface

• A. CORRECT. Read the tracing against the two critical pH thresholds. Salivary pH oscillates between roughly 5.6 and 6.1 — it never falls below 5.5, the critical pH for ENAMEL, but it sits continuously below 6.2, the critical pH for ROOT SURFACE (cementum and dentin). Root surface is the less mineralized and more soluble tissue, so it demineralizes at a higher pH. A patient with post-radiation xerostomia also has gingival recession and exposed root surfaces, giving the lesion a site to develop. Enamel is being spared; root surface is not.
• B. Anterior teeth are certainly involved in radiation caries, but again this names a region rather than a surface. The graph identifies which TISSUE is dissolving, and the answer must be the surface, not the segment.
• C. 'Posterior teeth' names a location in the arch, not a tooth surface, and the graph carries no information about arch position. Radiation caries characteristically affects the CERVICAL and incisal regions across the mouth rather than favouring the posterior segments.
• D. The occlusal surface is enamel. At no point does this patient's pH fall below the 5.5 enamel threshold, so occlusal enamel is not the tissue at risk in this tracing.

Note: Two critical pH values worth memorizing cold: ENAMEL / hydroxyapatite ≈ 5.5; ROOT SURFACE (dentin & cementum) ≈ 6.2; fluorapatite ≈ 4.5. Post-radiation xerostomia produces the classic pattern of cervical, circumferential 'radiation caries.' Management is aggressive prevention: 5000 ppm fluoride dentifrice, custom fluoride trays, saliva substitutes, and lifelong recall.