A 20-year-old patient complains about pain and hemorrhages in the region of the 36 tooth occuring during eating solid food. Objectively: medial masticatory surface of the 36 tooth has a large carious cavity occupied by a carneous tumour-like formation, probing induces hemorrhage and pain in the region of connection of the carious cavity with the pulp chamber. Percussion is painless. Electroodontodiagnosis is 40 microampere. Roentgenological changes are absent. What is the most likely diagnosis?