A 58-year-old Indian vegetarian woman presents with a 3-month history of progressive fatigue, paresthesias in both feet, and unsteady gait. Peripheral blood smear shows hypersegmented neutrophils (≥5 lobes) and macrocytic RBCs. The structure marked **C** in the anemia classification diagram represents the type of anemia most consistent with her presentation. Which of the following laboratory findings would BEST differentiate the underlying cause of the megaloblastic anemia marked **C** from folate deficiency?