A 53-year-old male presents with dizziness, fatigue, limb numbness, and unsteady gait for over 6 months. He underwent total gastrectomy 8 years ago due to gastric bleeding. Laboratory tests: RBC 2.1×10¹²/L, Hb 72g/L, WBC 3.2×10⁹/L, PLT 86×10⁹/L, MCV 140fl (140μm³), MCH 42pg, serum vitamin B₁₂ 86pg/ml (normal: 200-900pg/ml), normal folate. The diagnosis is vitamin B₁₂ deficiency anemia. The most likely cause is: