A newborn presents with progressive hydrocephalus. MRI shows cystic dilation of 4th ventricle due to blockage of foramina of Luschka and Magendie. Diagnosis?

Correct option is C

Dandy–Walker malformation involves agenesis of cerebellar vermis, cystic dilatation of 4th ventricle, and hydrocephalus due to blocked foramina of Luschka and Magendie.
Explanation of options: (a) ✘ Arnold–Chiari is downward herniation of cerebellum, not cystic 4th ventricle. (b) ✘ Holoprosencephaly = failure of forebrain division, not posterior fossa anomaly. (c) ✔ Correct; classic triad = hydrocephalus, enlarged posterior fossa, 4th ventricle cyst. (d) ✘ Aqueductal stenosis blocks 3rd–4th ventricle pathway, not 4th ventricle exit.