A 14-year-old patient complains of intense, throbbing pain in the region of the 46 tooth which appeared for the first time two days ago. Objectively: the 46 tooth is filled. Percussion causes acute pain, the tooth is mobile, the mucous membrane around the 46 tooth is hyperemic and edematous, palpation of the mucogingival fold in the root apex projection is painful. Ro-gram shows no changes in the periapical region of the 46 tooth. What is the most likely diagnosis?