A 58-year-old man with chronic liver disease and severe hyponatremia (Na 118 mEq/L) is admitted. His serum sodium is corrected at 12 mEq/L over 24 hours. On day 3, he develops progressive quadriparesis, dysphagia, and dysarthria. Axial MRI brain at the pons level shows a characteristic non-enhancing T2-hyperintense lesion in the central region of the structure marked **A**, sparing the periphery, with a "bat wing" appearance on imaging. Which of the following best explains this clinical and radiological presentation?