A 30-year-old man complains of a carious cavity in a lower left tooth. Objectively, tooth 34 has a deep carious cavity that at one point communicates with the pulp chamber. Probing of the cavity floor is painless, while percussion of the tooth is slightly painful. Thermal stimuli provoke no response. On the gums, there is a fistula in the projection of the root apex of tooth 34. Make the provisional diagnosis: