A 38-year-old woman from Mumbai presents with a 2-month history of progressive fatigue, dyspnea on exertion, and recurrent mouth ulcers. On examination, she has glossitis, angular cheilitis, and a mild systolic murmur. Her hemoglobin is 9.2 g/dL with macrocytic indices (MCV 108 fL). Serum B12 and methylmalonic acid are normal, but serum folate is 2.8 ng/mL (normal >5.4). She reports a vegetarian diet with minimal legume intake and takes phenytoin for epilepsy. What is the primary mechanism of her anemia?