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?

Explanation

## Why option 1 is correct Medulloblastoma is the most common malignant pediatric brain tumor and characteristically arises in the cerebellar vermis (the structure marked **B**). Vermis lesions produce truncal (axial) ataxia—the classic "drunken sailor" wide-based gait with falls in any direction and inability to sit/stand without support—because the vermis controls truncal and axial posture. The preserved finger-nose-finger test indicates sparing of appendicular coordination, which is typical of vermis pathology (as opposed to hemispheric lesions). The mass compressing the 4th ventricle explains the obstructive hydrocephalus and vomiting. This clinical triad (truncal ataxia, hydrocephalus, midline posterior fossa mass in a child) is pathognomonic for medulloblastoma. [Gray's Anatomy 42e Ch 22; standard pediatric neuro-oncology] ## Why each distractor is wrong - **Option 2 (Cerebellar hemisphere astrocytoma)**: Hemispheric lesions cause *ipsilateral appendicular* ataxia (intention tremor, dysmetria, dysdiadochokinesia), not truncal ataxia. The patient's preserved finger-nose-finger test rules this out. Astrocytomas are also less common than medulloblastoma in the posterior fossa of children. - **Option 3 (Flocculonodular lobe hemangioblastoma)**: Flocculonodular lobe (vestibulocerebellum) lesions cause vertigo, nystagmus, and vestibular ataxia—not truncal ataxia. There is no mention of vertigo or nystagmus in this case. Hemangioblastomas are associated with von Hippel–Lindau syndrome and are rare in children without that syndrome. - **Option 4 (4th ventricle ependymoma)**: While ependymomas can cause hydrocephalus, they do not directly cause truncal ataxia unless they secondarily compress the vermis. The clinical presentation is dominated by truncal ataxia with preserved appendicular function, which is specific to vermis pathology, not just ventricular obstruction. **High-Yield:** Cerebellar vermis lesions → truncal ataxia (wide-based gait, falls in all directions, impaired sitting/standing); medulloblastoma is the classic pediatric vermis tumor causing this triad: truncal ataxia + hydrocephalus + midline posterior fossa mass. [Gray's Anatomy 42e Ch 22]