An 11-year-old boy complains of a short-term pain from the cold in a left mandibular tooth. Objectively: the medial surface of the 36 tooth exhibits a carious cavity within parapulpar dentin. The cavity is filled with light, softened dentin and does not communicate with the cavity of the tooth. Probing the of the 36 tooth floor causes pain response, the tooth is not sensitive to percussion, the response to the cold stimulus does not stay long after its removal. What is the most likely diagnosis?