Parents of a 3-year-old child report that the child has persistent pain in the front maxillary teeth. Objectively: the coronal part of the 61 tooth is gray and decayed. Probing of the root canal orifice causes pain response and is accompanied by bleeding. The tooth percussion provokes acute pain. Mucosa is hyperemic, edematous and tender. Palpation in the region of the 61, 62 teeth reveals a fistula. What is your provisional diagnosis?