How do you radiographically distinguish internal from external root resorption?

Correct answer: D. Take two radiographs at different horizontal angles — an internal lesion stays centred on the canal

• A. Incorrect. Both appear radiolucent.
• B. Incorrect. Internal resorption most often affects the middle third of the canal and can occur anywhere.
• C. Incorrect. CBCT is very helpful and often definitive, but two-angle periapicals are the standard first-line method and are all that is needed in many cases.
• D. Correct. Using the SLOB/parallax principle: an INTERNAL lesion is continuous with the canal and remains superimposed on it regardless of beam angle, whereas an EXTERNAL lesion shifts away from the canal and the canal outline can be traced through it.