A 21-year-old patient complains of a constant progressing throbbing pain in the 27 tooth. Objectively: a large carious cavity is filled with softened dentin, the tooth cavity is closed. Probing of the cavity floor is painless, percussion causes acute pain, there is grade II tooth mobility. Palpation of the mucous membrane in the projection of the root apex of the 27 tooth causes pain. Radiological changes are absent. What is the most likely diagnosis?