A 65-year-old male patient complains of a long-lasting pain in a mandibular tooth on the right. The pain increases on biting. The patient has a history of this tooth treatment for caries. Objectively: the cavity on the masticatory surface of the 36 tooth is filled, percussion causes pain responce. Along the mucogingi-val junction in the projection of the 36 tooth roots there is a fistula with purulent discharge. What is the most likely diagnosis?