A 22-year-old woman presents with “the worst sore throat I’ve ever had.” She also complains of fatigue and anorexia. She is not immunocompromised and has not been hospitalized recently. On examination, she is febrile to 38°C, the pharynx is inflamed, and there are a few tender cervical nodes bilaterally. There are no white lesions on the tongue or pharynx. A throat culture grows alpha-hemolytic colonies on blood agar that are optochin-resistant. Of the following, which one is the MOST likely cause?