Parents of a 3-year-old child report that the child suffers from constant pain in the upper front teeth. Objectively: the coronal part of the 61 tooth is gray and decayed. Probing of the root canal orifice is painful and accompanied by bleeding. The tooth percussion provokes acute pain. Mucosa is hyperemic, edematic and painful. Palpation in the region of the 61 and 62 teeth reveals a fistula. What is your provisional diagnosis?