A 28-year-old male presents with dizziness, palpitations, fatigue for 1 year, accompanied by limb numbness and unsteady gait. Past medical history includes Crohn's disease. Physical examination reveals severe anemic appearance, heart rate 120 bpm, Romberg's sign (+), and heel-knee-shin test (+). Megaloblastic anemia is suspected. Which laboratory test is most definitive for diagnosis?