Dandy-Walker syndrome is a congenital malformation that affects the development of the cerebellum. Typical features: the vermis is absent or hypoplastic, cystic enlargement of the fourth ventricle, enlarged posterior fossa. May be associated with hydrocephalus (this patient has normal sized ventricles). This three month old’s brain ultrasound demonstrates posterior fossa enlargement with cystic dilation of the fourth ventricle. Also has a round fluid collection noted which is likely a cyst versus posterior horn dilation.