Which of the following is LEAST likely to cause endodontic failure?

Correct answer: B. Slight extrusion of gutta-percha beyond the apex

• A. Inadequate debridement leaves viable bacteria in the canal system — the single most common cause of endodontic failure.
• B. CORRECT — this is the least likely cause. A small amount of gutta-percha past the apex is an inert foreign body. Provided the canal beneath it has been thoroughly disinfected and densely obturated, healing usually proceeds normally; the excess is walled off or slowly resorbed. Overextension is a technical imperfection, not the principal cause of failure.
• C. A missed canal — most classically MB2 in a maxillary molar — leaves a wholly untreated infected space and is a leading cause of persistent disease.
• D. Coronal leakage recontaminates even a well-obturated canal. A definitive coronal restoration is as important to the outcome as the root filling.

Note: Distinguish overEXTENSION (material past the apex, length error) from overFILLING (excess material with the canal properly filled). The critical variable in both cases is whether the canal was adequately disinfected and sealed.