It's almost certain that if you have one of these in the options then the other will also be there!
Here's a quick reminder
Arnold Chiari:
a congenital anomaly in which a narrow craniospinal junction and a small posterior cranial fossa causes displacement of the cerebellum and medulla oblongata downward which leads to compression of the CSF passage at the level of foramen magnum. Two types, type I is asymptomatic and type II is symptomatic and commonly associated with meningomyelocele and syringomyelia.
Dandy Walker:
also congenital and also around the fourth ventricle but this time it's a small cerebellar vermis that give room to the fourth ventricle to expand in a cyst like structure which could also obstruct leading to hydrocephalus. Unlike Arnold, the posterior fossa is large.
Here's a quick reminder
Arnold Chiari:
a congenital anomaly in which a narrow craniospinal junction and a small posterior cranial fossa causes displacement of the cerebellum and medulla oblongata downward which leads to compression of the CSF passage at the level of foramen magnum. Two types, type I is asymptomatic and type II is symptomatic and commonly associated with meningomyelocele and syringomyelia.
Dandy Walker:
also congenital and also around the fourth ventricle but this time it's a small cerebellar vermis that give room to the fourth ventricle to expand in a cyst like structure which could also obstruct leading to hydrocephalus. Unlike Arnold, the posterior fossa is large.