A 15-year-old adolescent complains of a two-day long sharp pulsing pain in the tooth, which intensifies when biting or touching the tooth with the tongue. Objectively: the 26th tooth contains composite material filling. Vertical and horisontal percussion of the tooth are painful, the tooth is slightly mobile in vestibular-oral projection. Gingival mucosa of the affected area is hyperemic, swollen, sharply painful when palpated. X-ray imaging shows no changes. What is the most probable diagnosis?