PRES Posterior Reversible Vasogenic

A 29-year-old primigravid woman at 34 weeks presents with severe headache, visual disturbance (scintillating scotomas progressing to bilateral cortical blindness), and a witnessed generalized tonic-clonic seizure. On examination: BP 210/120, hyperreflexia with sustained clonus, brisk plantars, and RUQ tenderness. Labs show platelets 88, Hb 10.4 with schistocytes, LDH 620, AST 148, ALT 132, creatinine 108. MRI shows the abnormality marked **A** in the diagram — bilateral symmetric parieto-occipital cortical and subcortical FLAIR hyperintensities that are bright on ADC (vasogenic edema, not diffusion restriction). Which of the following best describes the underlying pathophysiology of the imaging finding marked **A**?