In a Class II preparation, which internal feature helps prevent marginal leakage?

• A. Rounded, wide internal angle.
• B. Clean internal angle.
• C. Deep grooves in the gingival floor.
• D. An internal bevel on enamel.

Answer: (B)

A clean internal angle is essential because, in a Class II amalgam preparation, a smooth, debris-free internal surface allows for proper condensation of the material against tooth structure and a tight seal at the margins. When the internal angle is clean, there are no irregularities or contaminants that could trap alloy or create voids, which are potential pathways for marginal leakage. Debris or roughness inside the box compromises adaptation and invites microleakage. While rounding internal angles can help reduce stress during condensation, and beveling can aid bonding in some restorative scenarios, the most direct way to prevent leakage in this context is ensuring the internal angle is clean so the amalgam can seat firmly and seal the margins effectively.