A 9-year-old girl presents with a two-week history of persistent sore throats, swallowing difficulty, and fatigue. She also has 38°C fever, a high erythrocyte sedimentation rate, and an elevated anti-streptolysin O titer. Tonsillar swabs were negative for group A Streptococcus. Biopsies of swollen lymph nodes show a mixed infiltrate of neutrophils and histiocytes that form structures resembling an owl's eye on electron microscopy. Based on these findings, which agent is most likely responsible for her condition?