A 25-year-old student complains of a carious cavity in the 22 tooth. The fillings fell out 2 months ago. The tooth had been treated before for pulpitis. Objectively: there is a deep carious cavity with the rests of filling on the medial surface of the 22 tooth. The crown of the 22 tooth is dirty pink. X-ray shows a root canal filled with the filling material by 1/2 of the root length; in the region of the root apex there is a well-defined focus of destruction of bone tissue 0,3x0,3 cm large. What is the most likely diagnosis?