Fetal Cystic Hygroma (Nuchal)

A 22-year-old primigravida at 13 weeks gestation undergoes first-trimester screening ultrasound. The midsagittal plane of the fetal neck shows a large anechoic fluid collection measuring 12 mm in the posterior nuchal region, far exceeding the normal limit of <3 mm at this crown-rump length. In the coronal and axial planes, the structure marked **A** appears as a large bilateral multiseptated cystic mass with internal septations and a prominent midline septum. Associated findings include generalized fetal skin edema, pleural effusions, and pericardial effusion. Chorionic villus sampling confirms 45,X karyotype. Based on the diagnosis of septated cystic hygroma with associated hydrops, what is the underlying pathophysiological mechanism responsible for the structure marked **A**?