A 7-year-old child complains about spontaneous pain in the upper right molars. Both medial contact and masticatory surfaces of the 55 tooth have a carious cavitiy filled with clear softened dentin and localized within parapulpar dentin. Floor probing causes acute pain, tooth percussion is slightly painful. Mucous membrane of the alveolar process in the root projection of the 55 is intact, examination of this region revealed no roentgenological changes. What is the most likely diagnosis?