A patient complains about acute constant pain that is getting worse when biting down on food, sensation of a recently erupted tooth on the right upper jaw. Examination of the 15 tooth revealed a deep carious cavity communicating with the dental cavity. Tooth percussion causes acute pain. Probing is painless. Mucous membrane in projection of the root apex is hyperemic, painful on palpation. X-ray picture shows no changes. What is the most likely diagnosis?