A 12-year-old boy complains about permanent intense throbbing toothache that is getting worse when biting down on food. Objectively: the patient's face is asymmetric because of a collateral edema of soft tissues, submandibular lymph nodes are enlarged and painful on palpation. The 26 tooth has a deep cavity not communicating with the tooth cavity. Thermal test is positive, probing is painless, percussion causes acute pain, the tooth is mobile. X-ray picture of the 26 tooth shows no changes in the periodontium. What is the most likely diagnosis?