A 77-year-old woman presents with headaches and difficulty walking. She has an unbalanced gait and falls easily, especially when trying to walk upstairs. The tone is normal and there are no cerebellar findings. A CT scan reveals enlarged ventricles.For the above patient with neurologic symptoms, select the most likely structural pathology

Adults may develop hydrocephalus as a result of occlusion of CSF pathways by tumors in the third ventricle, brain stem, or posterior fossa. In adults, the symptoms of obstructive hydrocephalus include headache, lethargy, malaise, incoordination, and weakness. Seizures do not usually occur. Dementia, altered consciousness, ocular nerve palsies, papilledema, ataxia, or corticospinal tract signs may be present.