A 40-year-old female patient complains of short-term pain in the 34th tooth caused by thermal stimuli. Objectively: the distal surface of the 34th tooth has a carious cavity localised in the circumpulpar dentin. The walls and floor of the cavity are softened, faintly pigmented; probing of the floor is slightly painful. Cold stimulus causes pain that ceases when stimulation stops. What is the most probable diagnosis?