<s>[INST] context: Midline posterior fossa cyst seen in dandy walker syndrome
• With agenesis of cerebellar vermis.
• May be associated with hydrocephalus.
Arnold-Chiari malformations are congenital herniations of hindbrain structures into the spinal canal.
• In type I Arnold-Chiari malformation, the cerebellar tonsils herniate into the foramen magnum;
• In type II Arnold-Chiari malformation, parts of the hindbrain, cerebellar vermis, and fourth ventricle herniate into the foramen. This second type is commonly associated with meningomyeloceles. (Other neonatal presentations associated with Arnold-Chiari malformation include hydrocephalus (because of cerebrospinal fluid obstruction) and brainstem dysfunction (causing stridor and poor swallowing). Milder cases may present later in life with spinal cord or cerebellar symptoms. “Dandy Walker malformation” is associated with hypoplasia of the cerebellum associated with an enlarged fourth ventricle.
“Holoprosencephaly” is a complex malformation of the brain such that only a single large monoventricular cerebral hemisphere exists.
“Porencephaly” is a severe cleft in the brain that allows the ventricles to communicate with the subarachnoid space. [/INST] question: Midline intra-cranial cyst is: | options: A: Choroid plexus cyst B: Dandy-Walker cyst C: Unilateral hydrocephalus D: Porencephalic cyst | answer: B </s>