A 48-year-old man with alcohol use disorder is brought to the hospital by his family after they found him confused and unable to recognize them. He stopped drinking 48 hours ago. On examination, he is disoriented to time and place, has coarse tremor, and is sweating profusely. His wife mentions he had a brief seizure episode 6 hours ago. Blood glucose is 95 mg/dL, serum sodium 128 mEq/L, serum magnesium 1.2 mg/dL. Which electrolyte abnormality is MOST directly responsible for his seizure risk in alcohol withdrawal?