A 38-year-old male patient complains of a carious cavity. He had experienced spontaneous dull pain in the tooth in question before. Objectively: the distal masticatory surface of the 37 tooth presents a deep cavity made of soft pigmented dentin. Percussion is painless. After removing the decay from the cavity, cold water has caused pain lasting for about a minute. X-ray picture shows the deformation of the periodontal gap in the region of the 37 root apices. What is the most likely diagnosis?