During a regular visit to the pediatrician 1 week after birth, an infant's size and head circumference are recorded as being in the seventy-fifth percentile. Repeat measurement 1 month later still shows the size of the baby at the seventy-fifth percentile, but the baby's head circumference is now at the ninety-fifth percentile. The pediatrician notices that the baby's anterior fontanelle is tense and that the skull sutures are open. He obtains an MRI of the brain with intravenous contrast. This study shows the presence of greatly dilated lateral and third ventricles. The aqueduct of Sylvius cannot be easily visualized. The fourth ventricle is small. There are no lesions within the subarachnoid space or cerebral parenchyma. The appearance of the MRI is consistent with which of the following?