A 45-year-old patient complains of acute, spontaneous, nocturnal pain in a maxillary tooth on the right. The pain arose three days ago, has a pulsating nature, irradiates to the zygomatic region and virtually doesn't cease. Objectively: there is a deep carious cavity in the 15 tooth. Dentin is of dirty-gray color, softened. Probing causes pain response across the entire floor, the tooth is sensitive to percussion. Cold stimulus reduces the pain intensity. Electroexcitability of the pulp is 55 iA. What is the most likely diagnosis?