A 38-year-old woman from rural Maharashtra presents with progressive paresthesias in both feet, difficulty walking, and loss of vibration sense in the lower limbs. She reports a strict vegetarian diet with no dairy or egg intake for the past 8 years. Serum cobalamin level is 180 pg/mL (normal >200). Which investigation would be most specific for confirming vitamin B12 deficiency and differentiating dietary from malabsorption causes?