A 6-month-old child was at the 50th percentile at birth for length, weight, and head circumference. His growth curve from his last visit 1 week ago is shown. On his developmental assessment, you noted that he rolled from stomach to back occasionally but not very well from back to stomach. He could bear weight on his legs but would not sit without assistance. Today, the family calls you urgently at 7:00 AM noting that their child seems unable to move the right side of his body. Which of the following conditions might explain this child's condition?