A 44-year-old woman with chronic alcohol use disorder (200 g ethanol daily) presents with 4 days of vomiting, diarrhea, and reduced oral intake. She is found to have severe hyponatremia (sodium 108 mmol/L) with signs of malnutrition, hypokalemia (2.6 mmol/L), and hypophosphatemia. Despite guideline recommendations, her sodium is corrected too rapidly—rising 20 mmol/L in 24 hours due to over-aggressive hypertonic saline and brisk aquaresis. Over the next 48–72 hours she develops quadriparesis, dysarthria, dysphagia, and gaze palsies progressing to locked-in state. MRI brain 5 days later shows the finding marked **A** in the diagram. Which of the following best describes the pathophysiology of the MRI abnormality at **A**?