A 19-year-old university student presents with a 5-day history of fever, severe sore throat, malaise, and headache. On examination, temperature is 38.5°C, pharynx is erythematous with exudates, and significant bilateral cervical lymphadenopathy is noted. Palatal petechiae are visible. Abdominal examination reveals mild splenomegaly. A blood film report requested by the GP mentions 'atypical lymphocytes'. What is the most likely causative organism?