A 28-year-old second-gravida with a history of intrauterine infection presents at 26 weeks of gestation with rapidly increasing fundal height. Transabdominal fetal ultrasound with transventricular axial view shows the finding marked **A** in the diagram: symmetric massive dilatation of both lateral ventricles (atrial widths 18–20 mm), thinned cortical mantle (<3 mm), and choroid plexuses hanging freely within the fluid-filled ventricles. Detailed neurosonography reveals no primary CNS malformation or spinal defect. Fetal MRI confirms aqueductal stenosis with normal posterior fossa. Which of the following is the most likely pathophysiological mechanism underlying this presentation?