A week ago an 18-year-old girl complained of pain in the 22 tooth which was treated and filled several years ago. Over the past two days the pain increased. Objectively: the 22 tooth is filled, percussion is painful, mucous membrane is hyperaemic and edematic. Spot-film roentgenograph of the 22 tooth shows an ill-defined periapical bone rarefaction 0,4x0,5 cm large. What is the most likely diagnosis?