A 35-year-old man complains of persisting pain in tooth 24, which intensifies on biting. Objectively on the distal masticatory surface of tooth 24 there is a deep carious cavity filled with food debris. Percussion of cavity bottom is painless, there is no pain response to thermal stimuli. Percussion of tooth 24 is acutely painful: X-ray shows no pathologic changes of periapical tissues in the area of root apices of 24. What is the most likely diagnosis?