A 68-year-old man with chronic malabsorption due to celiac disease presents with a 6-month history of progressive muscle weakness, ataxia, and loss of vibration sense in the lower limbs. He reports easy bruising and epistaxis for the past 2 months. Laboratory investigations reveal: INR 2.8 (normal: 0.8–1.1), aPTT 45 seconds (normal: 25–35 seconds), serum α-tocopherol 2.1 mg/L (normal: 5.4–17.2 mg/L). Prothrombin time corrects with fresh frozen plasma but not with vitamin K injection. Which vitamin deficiency best explains the neurological findings?