Which condition features hypoplasia of the cerebellar vermis with expansion into a large cystic structure of the fourth ventricle in the posterior fossa?

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This question tests recognition of a classic posterior fossa malformation where the cerebellar vermis is underdeveloped and the fourth ventricle expands into a large cyst. That combination defines Dandy-Walker malformation. In this condition, vermian hypoplasia or agenesis pairs with cystic dilation of the fourth ventricle, enlarging the posterior fossa and often elevating the tentorium. On imaging you’d expect to see a prominent cystic area where the fourth ventricle should be, along with an enlarged posterior fossa. This differentiates it from other posterior fossa anomalies: Arnold-Chiari malformation involves downward displacement of the cerebellar tonsils and brainstem rather than vermian hypoplasia with a fourth-ventricle cyst; Joubert syndrome shows midbrain-hindbrain malformations with the molar tooth sign; Chiari II malformation is typically associated with a myelomeningocele and a beaked tectum.