A crown will not seat completely on the preparation. Which of the following is LEAST likely to be responsible?

Correct answer: A. Lack of occlusal reduction on the preparation

• A. CORRECT — this is the exception. INSUFFICIENT OCCLUSAL REDUCTION produces a crown that is too THIN occlusally or that sits HIGH in occlusion once seated. It does not prevent the crown from seating on the preparation, because the occlusal surface is not a seating interference — the restoration still drops fully onto the axial walls and margin.
• B. TIGHT PROXIMAL CONTACTS are one of the most common reasons a casting will not go down. The remedy is to identify the binding contact with thin articulating film or floss and relieve it before assuming the fault lies internally.
• C. POSITIVE NODULES on the intaglio surface — from air bubbles in the impression or investment, or from debris — are classic seating interferences. Disclosing medium (silicone spray, powder, or a disclosing wax) applied to the intaglio identifies exactly where the casting is binding so it can be relieved.
• D. RESIDUAL TEMPORARY CEMENT is a frequent and easily overlooked cause. The preparation must be thoroughly cleaned — hand instruments, pumice, air-water spray — before try-in.

Note: Systematic approach when a crown will not seat: clean the preparation, check proximal contacts, then use a disclosing medium on the intaglio and relieve only where it marks. Never simply force the casting or grind the margin.