A 48-year-old man complains of a dull pain in the area of his tooth 48 that intensifies when biting, worsening of his general condition, and high body temperature of 38°C. Objectively, the retromolar mucosa is edematous and hyperemic. The distal surface of tooth 48 is covered with a mucosal flap, palpation of which provokes a sharp pain and discharge of purulent exudate. What is the most likely diagnosis in this case?