An 18-year-old young man complains of a cavity in his tooth 37 and pain that occurs when food gets into this cavity. The tooth was painful one year ago but he did not make an appointment with the doctor back then. Objectively, on the masticatory surface of tooth 37 there is a deep carious cavity that communicates with the pulp chamber. Overgrown granulation tissue protrudes from the perforation into the pulp chamber and fills practically the whole carious cavity. Probing of the granulations provokes bleeding and slight pain. Percussion is painless. Mucosa in the projection of the roots of tooth 37 is pale pink and has no pathologic changes. Electric pulp testing-60 microamperes. What is the most likely diagnosis?