A 28-year-old primigravida from Delhi presents to the obstetric ultrasound clinic at 20 weeks of gestation for a routine anomaly scan. On transabdominal ultrasound, the fetus shows a large fluid-filled cavity in the posterior fossa measuring 12 mm in anteroposterior diameter, with normal cerebellar hemispheres bilaterally and an intact vermis. The lateral ventricles measure 10 mm. Nuchal fold thickness is 5 mm. What is the most likely diagnosis?