Final answer:
The patient's symptoms indicate an irreversibly inflamed pulp due to a deep carious lesion, as evidenced by the sharp, stimulus-stopped pain and a higher response to electric pulp testing.
Step-by-step explanation:
The patient's symptoms suggest that the most probable diagnosis for the condition of the pulp and the apical periodontal ligament is a irreversibly inflamed pulp. The tooth structure includes the crown covered with enamel, underneath which is the dentin that surrounds the pulp cavity. The pulp within this cavity is rich in blood vessels and nerves and responds to the thermal stress indicated by the sharp pain upon application of heat or cold, which stops after the stimulus is removed. The fact that the pain is not ongoing suggests that the pulp is not necrotic, as that would typically not respond to an electric pulp test or show pain. Since the apical periodontal ligament appears normal on radiographs and there is no sensitivity to percussion or palpation, it is unlikely to be inflamed. Therefore, the deep, distal carious lesion found on the radiographs likely led to the inflamed pulp condition, which is further supported by the higher threshold response to electric pulp testing compared to the control tooth.