An 18-month-old boy is brought to the emergency department by his parents, who immigrated from a country with low routine vaccination coverage. Three days ago he developed fever to 38.9°C and fussiness; the fever resolved yesterday and he seemed to be improving. This morning his parents noticed he was not pulling himself to stand and cried when they moved his left leg. His vaccinations are not up to date.
On examination he is alert and interactive. Left lower extremity strength is 0/5 proximally and distally; left patellar and Achilles reflexes are absent. Right lower extremity strength is 5/5 with intact reflexes. Light touch, pinprick, and proprioception are intact in both lower extremities. Lumbar puncture shows: WBC 92 cells/µL (91% lymphocytes), glucose 58 mg/dL (serum glucose 74 mg/dL), protein 52 mg/dL.
Which of the following most likely caused this child's acute lower extremity findings?