Which of the following most likely causes a com¬municating (nonobstructive) hydrocephalus?

Blockage of the arachnoid granulations would most likely cause a communicating, or nonobstructive, hy­drocephalus. Hydrocephalus is an increase in cerebrospinal fluid (CSF) volume with distention of the ventricles. It may be caused by (1) overproduction of CSF, which is uncommon (e.g., in neoplasms of the choroid plexus); (2) decreased absorption of CSF from the arachnoid villi that is caused by an organizing meningitis with fibrosis blocking the arachnoid granulations, dural si­nus thrombosis, or organization of blood clot in a subarachnoid hemorrhage; or (3) obstruction of CSF flow within the ventricles with distention of the ventri­cles proximal to the block, in conditions such as a colloid cyst of the third ventricle, stenosis of the duct of Sylvius, tumor (e.g., ependymoma, medulloblas­toma) in the fourth ventricle, or the blockage of flow out of the foramina of Luschka and Magendie by pus (tuberculous meningitis) or blood. Blockage of CSF flow within the ventricles produces an obstructive, or noncommunicating hydrocephalus. Blockage of the resorption of CSF out of the subarachnoid space pro­duces a communicating hydrocephalus, because there is still an open communication between the ventricles and the subarachnoid space. In any of these situations, the ventricles dilate under the increased pressure and are easily identified by computerized tomography scans or magnetic resonance imaging.