All of the following can help determine how many roots or canals a tooth has EXCEPT:

Correct answer: B. An electronic apex locator

• A. CBCT is the most accurate method available for canal anatomy and is specifically indicated for suspected extra canals in retreatment cases.
• B. CORRECT — this is the EXCEPT. An apex locator measures the WORKING LENGTH of a canal you have already found and instrumented. It gives you no information about how many canals exist, and it cannot find a canal you have missed.
• C. Shifting the horizontal angle (the SLOB rule) separates superimposed roots and is the classic chairside method for identifying a second canal.
• D. The pulp chamber floor map — the developmental grooves and the darker dentin lines connecting orifices — reliably indicates canal number under the microscope.

Note: Chamber floor law: canal orifices always lie at the junction of the walls and the floor, and the floor is always darker than the walls. If you cannot find a canal, look at the colour change, not the middle of the floor.