A 6-year-old patient complains of a long-lasting pain in a lower right tooth that is getting worse when biting down on food. The tooth has been previously treated for caries. Objectively: the cavity on the occlusal surface of the 36 tooth is filled, percussion is painful. Along the mucogingival fold in the projection of the 36 tooth roots there is a fistula with purulent discharge. What is the most likely diagnosis?