Where is a D2 interproximal caries located in the dentin?

• A. Outer third
• B. Pulp chamber
• C. Inner third
• D. Middle third of dentin

Answer: (D)

Dentin is described in terms of thirds—outer, middle, and inner—to indicate how far a caries lesion has penetrated beyond enamel. When a lesion reaches the middle third of dentin, it has progressed past the outer portion but has not yet intruded into the inner third near the pulp. So an interproximal caries located in the dentin at this depth sits in the middle third. This depth signals moderate progression, guiding treatment decisions that are more extensive than those for outer dentin involvement but not yet as deep as near-pulp dentin.