How does the isthmus of an amalgam preparation differ from that of a cast gold inlay preparation?

Correct answer: B. The amalgam isthmus converges occlusally; the inlay isthmus diverges occlusally

• A. The inlay does not converge — a converging preparation would make a rigid casting impossible to seat.
• B. CORRECT. The difference follows directly from how each restoration is placed. AMALGAM is condensed INTO the preparation as a plastic mass, so the walls CONVERGE occlusally to lock it mechanically in place. A CAST INLAY is fabricated outside the mouth and must be seated along a single path of withdrawal, so its walls must DIVERGE occlusally, conventionally by 2–5°.
• C. This reverses both designs.
• D. The amalgam preparation does not diverge; divergence would allow the restoration to displace occlusally.

Note: The governing principle: plastic materials that are placed in bulk need UNDERCUT retention. Rigid materials fabricated indirectly need a PATH OF WITHDRAWAL, which forbids undercuts entirely.