PRES Syndrome

A 32-year-old woman at 35 weeks gestation with severe preeclampsia presents with sudden-onset headache, confusion, cortical blindness, and a generalized tonic-clonic seizure. Blood pressure is 210/120 mmHg. MRI brain shows bilateral symmetric T2/FLAIR hyperintensities in the parietooccipital white matter without restricted diffusion. EEG findings are shown in the diagram. The pattern marked **B** (bilateral posterior delta slowing with occasional focal sharp waves, reversible after blood pressure control) is most consistent with which underlying pathophysiological mechanism in PRES?