A 38-year-old male patient complains of carious cavity. Prior to that there were attacks of toothache with no external cause. Objectively: the distal masticatory surface of the 37th tooth has deep carious cavity filled with softened pigmented dentine. Percussion is painless. Upon the extraction of the degenerated tissue cold water stimulus causes pain lasting about 1 minute. X-ray imaging shows deformation of the periodontal fissure in the area of the root apexes of the 37th tooth. What is the most probable diagnosis?