A 6-year-old unimmunized boy from rural India presents to the pediatric ICU with a 5-day history of jaw stiffness followed by involuntary facial grimacing. On examination, the structure marked **A** in the diagram is evident—a characteristic sustained contraction of the facial muscles giving a peculiar grinning appearance. The child also has trismus and opisthotonus. Which of the following best explains the pathophysiology underlying the clinical sign marked **A**?