Start practicing

Endodontics · single question

In approximately what proportion of maxillary first molars can a fourth canal (MB2) be located?

  1. About 10%
  2. The great majority — laboratory and microscope studies report over 90%
  3. About 30%
  4. Fewer than 5%
Show answer and explanation

Correct answer: B. The great majority — laboratory and microscope studies report over 90%

  • A. ❌ 10% badly understates it and reflects only what is visible on a plain periapical radiograph.
  • B. ✅ CORRECT — Prevalence depends entirely on the detection method. Micro-CT and careful microscope-assisted clinical studies find an MB2 in over 90% of maxillary first molars; pooled CBCT data give roughly 70%. The practical lesson is the same either way: assume the MB2 is there and go looking for it.
  • C. ❌ 30% is well below even the most conservative pooled estimates for first molars.
  • D. ❌ Under 5% is not supported by any modern study of the first molar.

Note: ⚑ The source keys 95%. Cited figures range from ~46% (CBCT, some populations) to ~97% (micro-CT). Reported prevalence rises with magnification. A missed MB2 is a leading cause of maxillary molar endodontic failure.

Practice Endodontics questions Endodontics study notes

More Endodontics practice questions