A 55-year-old man with 25 years of chronic heavy alcohol use and poor nutritional intake presents with acute confusion, wide-based gait ataxia, and ophthalmoplegia. MRI brain with T2 and FLAIR sequences is performed. The structure marked **A** shows symmetric hyperintensity in the mammillary bodies, along with similar signal changes in the periaqueductal gray matter, medial thalami, and floor of the fourth ventricle. Which of the following best explains the selective vulnerability of the structure marked **A** in this clinical presentation?