A 34-year-old woman at 32 weeks gestation presents with severe headache, visual disturbance, altered mental status, and a generalized tonic-clonic seizure. Blood pressure is 210/120 mmHg with proteinuria and hyperreflexia. MRI brain demonstrates bilateral symmetric hyperintensities in the subcortical white matter and cortex of the parietal and occipital lobes, as marked by **A** in the diagram. Which of the following best explains why the posterior circulation is preferentially affected in this condition?