Which tooth is MOST likely to present with four canals?

Correct answer: D. Maxillary first molar

• A. The mandibular second premolar usually has a single canal; multiple canals occur in a minority of cases.
• B. The mandibular first molar typically has three canals (two mesial, one distal); a fourth distolingual canal occurs but far less often than MB2.
• C. The maxillary second molar can have MB2, but at a substantially lower frequency than the first molar, and its roots are often fused.
• D. CORRECT. The maxillary first molar has three roots but commonly FOUR canals, because the mesiobuccal root frequently contains a second canal (MB2). Reported MB2 incidence ranges from roughly 60% to over 90% with magnification and ultrasonics. A missed MB2 is one of the most common causes of endodontic failure in this tooth.

Note: Always assume MB2 exists in a maxillary first molar until you have proven otherwise under magnification.