A 4-year-old child complains of acute spontaneous pain in a mandibular tooth on the right. The pain is aggravated by cold stimuli. Objectively: the 84 tooth exhibits a deep carious cavity that does not communicate with the tooth cavity. Probing causes pain response all over the cavity floor, percussion causes pain. The mucosa in the region of the 84 tooth is hyperemic, edematous. Regional lymph nodes are enlarged. What is the most likely diagnosis?