A woman complains of a severe pain in her throat on the left, difficult swallowing and mouth opening, elevated body temperature, and generally feeling unwell. She was provisionally diagnosed with acute pharyngitis. Examination detects a trismus of the masticatory muscles, the left tonsil is displaced towards the midline, the anterior palatal arch is infiltrated and protruding. The regional lymph nodes on the left are enlarged and painful to palpation. What is the most likely diagnosis in this case?