A 3 year old boy is brought to the emergency depament after the acute onset of headache, vomiting, nuchal rigidity, and impaired mental status. MRI reveals a posterior fossa tumor that fills the 4th ventricle. Surgery is immediately staed, and intraoperative consultation leads to a "frozen section" diagnosis of medulloblastoma. Which of the following pathologic mechanisms most likely accounts for this child's clinical presentation?

Any tumor "filling the 4th ventricle" blocks the circulation of cerebrospinal fluid (CSF). This blockage leads to increased intracranial pressure, which manifests with nausea, vomiting, headache,nuchal rigidity, and mental status changes. If surgery is not performed promptly, cerebellar tonsillar herniation and rapid death will ensue. In children, medulloblastoma and ependymoma are the most frequent neoplasms presenting in this manner. There is no evidence in this case suggesting that acute hemorrhage into the 4th ventricular cavity has occurred, nor is medulloblastoma typically associated with this complication. CNS tumors that frequently bleed are metastases from melanoma, renal cell carcinoma, and choriocarcinoma. Alterations in medullary function lead to cardiorespiratory instability and may be caused by direct tumor compression or infiltration of the medulla, neither of which is suppoed by MRI findings in this case. Infiltration of the cerebellar vermis is ceainly seen in many cases of medulloblastoma, a tumor that arises from this midline cerebellar structure. However, this would lead to truncal ataxia and gait instability, not symptoms of increased intracranial pressure. Ref: Ropper A.H., Samuels M.A. (2009). Chapter 30. Disturbances of Cerebrospinal Fluid and Its Circulation, Including Hydrocephalus, Pseudotumor Cerebri, and Low-Pressure Syndromes. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor's Principles of Neurology, 9e.