A 52-year-old woman complains of periodical appearance of a gingival fistula in the area of the 15 tooth. The tooth had been treated 1,5 years ago for caries. Objectively: the 15 tooth is filled. In the root apex projection there is a fistula; purulent exudate discharges on pressure. Tooth percussion is painless. On X-ray: the root canal is not filled, there is a destruction focus with blurred margins near the root.Make the diagnosis: