A conical carious lesion with a broad base at the tooth surface and its apex directed toward the enamel-dentine junction is characteristic of:

Correct answer: D. Smooth surface caries

• A. Root caries has no enamel to traverse; it spreads laterally and circumferentially in cementum and dentine, typically as a shallow saucerised lesion.
• B. Secondary caries occurs at the restoration margin and does not have this characteristic conical form.
• C. Pit-and-fissure caries has the opposite orientation — a narrow apex at the surface and a broad base at the enamel-dentine junction.
• D. CORRECT. SMOOTH SURFACE caries — typically proximal — begins as a broad area of plaque-mediated demineralisation and then follows the direction of the enamel rods inward, which converge as they approach the enamel-dentine junction. The result is a triangular lesion with its BASE at the surface and its APEX pointing inward, the reverse of the pit-and-fissure pattern.

Note: Memorise the pair: SMOOTH surface = base out, apex in. PIT and fissure = apex out, base in. Both then invert on entering dentine, with the apex directed toward the pulp.