A 21-year-old university student attends the Emergency Department with a 3-day history of severe sore throat, painful swallowing, and fever. He reports reduced oral intake because swallowing is uncomfortable. He has no significant past medical history and takes no regular medication.

On examination, he is alert and speaking in full sentences. Temperature is 38.4°C. There is tender cervical lymphadenopathy. Oropharyngeal examination shows an erythematous pharynx with patchy exudate. The accompanying image is provided :

What is the most likely diagnosis?