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?