A 25-year-old man complains of brief pain attacks in response to sweet foods in one of his lower teeth and food retention in a carious cavity. Objectively, on the masticatory surface of tooth 36 there is a carious cavity within the parapulpar dentin. The dentin is soft. The carious cavity does not communicate with the dental cavity, probing of its floor is painful. Percussion of tooth 36 and palpation of gingival mucosa in the projection of root apex are painless. Thermal stimuli cause pain that quickly stops after the simulus is removed. What is the most likely diagnosis?