A 25-year-old patient complains of pain when biting on the 15 tooth. The pain arose two days ago, has a constant aching nature and increased significantly over the last day. Objectively: the crown of the 15 tooth is gray, the medial contact surface exhibits a deep carious cavity communicating with the tooth cavity. Percussion causes acute pain, the gingival mucosa in the projection of the 25 tooth root apex is hyperemic. The regional lymph node is tender. Radiograph shows an ill-defined zone of periapical bone destruction. What is the most likely diagnosis?