A 16-year-old boy with no prior medical problems has complained of headaches for the past 9 months. There are no abnormal findings on physical examination. CT scan of the head shows an enlargement of the lateral cerebral ventricles and third ventricle. A lumbar puncture is performed with normal opening pressure, and clear CSF is obtained, which has a slightly elevated protein, normal glucose, and no leukocytes. Which of the following intracranial lesions is most likely to cause these findings?

This is noncommunicating hydrocephalus with obstruction below the level of the third ventricle. If hydrocephalus had been present at birth, there would be increasing head size because the sutures are not yet closed, and congenital aqueductal stenosis would be suspected. At his age, a neoplasm should be suspected, and ependymomas arise in the ventricular system, often in the fourth ventricle, to cause obstruction of CSF flow. The increased CSF protein comes from this tumor, but the shedding of cells from the mass into the CSF is unlikely. Except for vascular malformation, the other options are uncommon at his age. An abscess is typically accompanied by fever, and most would be located in the cerebral hemispheres away from the ventricular system. Cryptococcal meningitis is accompanied by fever, and exudate can be found within the ventricular system and subarachnoid space, but there is more likely to be cerebral edema, not hydrocephalus. The demyelinating plaques of multiple sclerosis are small and do not usually act as mass lesions. Vascular malformations usually arise in the cerebral hemispheres.