A 5-year-old fully immunized girl developed fever and sore throat, and

on examination the doctor observed white exudates on both tonsils. As a result,

he made a tentative diagnosis of pharyngotonsillitis. He swabbed the tonsillar

exudates and 24 hours later, cultures grew Gram-positive cocci in long chains. The

bacterium was beta-haemolytic on blood agar.

Which is the most likely pathogen which caused the disease?