During the dental care procedures a 4-year-old child has been found to have a carious cavity in the 64 tooth. The cavity filled with softened dentin is located within parapulpal dentin and doesn't communicate with the tooth cavity. Probing of the cavity floor is painless. Gingival mucosa in the projection of the root apices of the 64 tooth exhibits a healed fistula. What is the most likely diagnosis?