A 43-year-old patient complains of a sharp pulsing pain in the area of the right upper jaw. The pain appeared three days ago. Objectively, an inflamed round infiltration can be palpated on the vestibular surface of gingival mucosa of tooth 26, closer to the gingival margin. The tooth is intact, with II-degree mobility. Its horizontal and vertical percussion is painful. Periodontal pockets are 4-5 mm deep. What is the most likely diagnosis?