Neural Tube Defects Myelomeningocele/Spina Bifida Practice Test

In Chiari II malformation, hindbrain structures herniate downward through the foramen magnum, which crowds and obstructs the passage where CSF exits the fourth ventricle. CSF normally flows from the fourth ventricle into the subarachnoid space via its outlets (the foramina of Magendie and Luschka) and then around the brain and spinal cord. When this outflow is blocked at the level of the foramen magnum, CSF cannot leave the fourth ventricle effectively, leading to accumulation and hydrocephalus.

Blocking the aqueduct of Sylvius would interrupt CSF flow higher up, between the third and fourth ventricles, which is not the characteristic disruption in Chiari II. Obstruction at the foramina of Monro would affect drainage from the lateral ventricles into the third ventricle, again a different site. Blockage at the choroid plexus would alter CSF production rather than outflow. Therefore, the best fit is obstruction of CSF release from the fourth ventricle with disrupted flow through the foramen magnum.