A necrotic tooth begins bleeding after completion of endodontic cleaning and shaping. What is the most likely cause?

Correct answer: B. Overinstrumentation into periapical tissue

• A. A hypochlorite accident produces sudden severe pain, swelling and ecchymosis from extrusion of NaOCl — not simply 'bleeding,' and it is unrelated to a tooth being necrotic.
• B. Correct. A truly necrotic tooth has no vital pulp to bleed. Fresh bleeding means instruments have passed THROUGH the apex into vital periapical tissue — i.e., over-instrumentation.
• C. Incomplete pulpectomy (retained vital pulp) applies to a VITAL tooth; this tooth is necrotic.
• D. Retained apical tissue would not produce active bleeding in a non-vital tooth.

Note: This corrects the document's marked answer (A). For a necrotic tooth, bleeding indicates passage past the apex = over-instrumentation.