A 14-year-old child complains about acute spontaneous pain in an upper jaw tooth on the right. The pain has been lasting for 3 days, it is throbbing, irradiating to the temple, getting worse at night. Objectively: surface of the 15 tooth exhibits a carious cavity within parapulpar dentine. Dentine is softened, of greyish colour. Probing of the whole cavity floor is painful, percussion of the 15 tooth is painless. What is the most likely diagnosis?