Which item is NOT one of the five primary resistance form features?

• A. Dentin thickness
• B. Remove undermined enamel
• C. Flat pulpal floor
• D. Rounded internal line angles

Answer: (A)

Resistance form focuses on designing the cavity so the tooth–restoration complex can stand up to biting forces without fracture. The features that define this form are about shaping the preparation to avoid stress concentrations and provide stable support for the restoration: eliminating undermined enamel so only sound tooth structure remains, creating a flat pulpal floor to distribute occlusal forces evenly, and rounding internal line angles to remove sharp corners that would concentrate stress. Dentin thickness, while critically important for overall strength, is a general structural prerequisite rather than one of the specific features used to define resistance form. So, dentin thickness is not listed among the five primary resistance form features, making it the best choice for NOT belonging to that set.