A patient complains about paroxysmal upper jaw toothache on the left that is getting worse at night. Toothache intensifies also under stimulation and irradiates to the left eye and temple. Similar attacks were noted three months ago, the patient didn't undergo any treatment. Objectively: the 25 tooth has a deep carious cavity communicating with the tooth cavity. Probing causes acute pain at the point of communication, vertical percussion is slightly painful, horizontal one is painless. Mucous membrane in the projection of root apex of the 25 tooth is unchanged, its palpation is painless. Thermal probe causes acute pain, the pain attack is long- lasting. Electroodontodiagnosis is 60 microampere. X-ray picture shows slight widening of periodontal fissure at the root apex of the 25 tooth. What is the most probable diagnosis?