A 7-year-old boy presents with progressive truncal ataxia, wide-based gait, and recent-onset vomiting. On examination, he cannot sit or stand without support and tends to fall in all directions, but his finger-nose-finger test is relatively preserved. Imaging shows a midline posterior fossa mass compressing the 4th ventricle. The structure marked **B** in the diagram is the primary site of pathology. Which of the following best explains his clinical presentation?