A 7-year-old child complains of spontaneous pain in the upper right molar teeth. Examination of medial contact and masticatory surfaces of the 55 tooth revealed a carious cavity composed of softened light dentin and localized within circumpulpar dentin. Floor probing is acutely painful, tooth percussion is slightly painful. Mucous membrane of the alveolar process in projection of 55 tooth roots is intact, there are no roentgenological changes in this area. What is the most probable diagnosis?