Final answer:
External cervical root resorption is the most probable complication after bleaching a poorly obturated tooth due to the irritative potential of bleaching agents on periodontal tissues.
Step-by-step explanation:
The most probable postoperative complication of bleaching a tooth that has not been adequately obturated is external cervical root resorption (E). Bleaching agents can penetrate the tooth structure and reach the cementum and periodontal ligament, causing inflammatory resorption, especially if the protective barrier of the obturation is insufficient. It is important to ensure proper obturation before bleaching to minimize the risk of this complication.
Postoperative complications like fracture, discoloration, retrograde pulpitis, or acute apical periodontitis can occur but are less probable in this context. External cervical root resorption is a known risk when the seal provided by appropriate obturation is missing, as the bleaching agents can then irritate the periodontal tissues.