Final answer:
Medulloblastoma is a brain tumor that obstructs the fourth ventricle in the brain stem and leads to various symptoms like headaches and increased pressure.
Step-by-step explanation:
Medulloblastoma is a type of brain tumor that arises in the cerebellum and obstructs the fourth ventricle. The fourth ventricle is a part of the ventricular system in the brain stem that opens into the subarachnoid space through the median and lateral apertures. This obstruction can cause symptoms like headache, dysfunction of the cranial nerves, and increased intracranial pressure.