During treatment of tooth #17 the canals have been cleaned and shaped and irrigated. On drying, the mesiobuccal canal bleeds and the patient reports pain. What does this indicate?

Correct answer: C. Vital pulp tissue remains in that canal

• A. Over-instrumentation causes pain and bleeding too, but a working length radiograph would show it. In a molar with an inaccessible canal, retained vital tissue is far more common.
• B. A perforation bleeds, but the bleeding comes from the perforation site rather than the canal orifice, and the file path deviates from the expected root anatomy on a radiograph.
• C. CORRECT. Bleeding plus PAIN from a canal that should have been fully debrided means live, innervated tissue is still there — most often an unlocated second canal (MB2) or an untreated apical remnant or a lateral canal branch. The answer is to re-examine the anatomy, extend the access, and complete the debridement under anaesthesia.
• D. A hypochlorite accident produces immediate severe pain, rapid swelling and often ecchymosis — a dramatic and unmistakable event, not simple bleeding on drying.

Note: Retained vital tissue is the leading cause of persistent pain between endodontic visits. In maxillary first molars MB2 is present in roughly 60 to 90% of cases; in mandibular molars, look for a middle mesial canal in the isthmus.