What is the single most important factor determining pulpal protection beneath a restoration?

Correct answer: D. The remaining dentine thickness

• A. Liners matter chiefly where remaining dentine is already thin — they supplement dentine, they do not substitute for it.
• B. Shade has no bearing on pulpal protection.
• C. Bonding agents seal tubules and reduce postoperative sensitivity, but they are secondary to the bulk of dentine remaining.
• D. CORRECT. REMAINING DENTINE THICKNESS is the dominant variable. Dentine is the pulp's own insulator, and its protective effect rises steeply with thickness: at about 2 mm, pulpal response to most restorative procedures and materials is negligible; below roughly 0.5 mm, the tubules are wide and numerous and the pulp is vulnerable to thermal, chemical and bacterial insult. No liner compensates for dentine that is not there.

Note: The tubule gradient explains it: near the pulp the tubules are wide and densely packed; near the enamel-dentine junction they are narrow and sparse. Deep dentine is therefore far more permeable than superficial dentine.