A 25-year-old female farmer presents with pallor and fatigue for 1 year. CBC shows: RBC 2.0×10<sup>12</sup>/L, Hb 50g/L, WBC 7.6×10<sup>9</sup>/L (neutrophils 50%, lymphocytes 26%, eosinophils 14%), serum ferritin 10μg/L. Blood smear reveals increased central pallor of RBCs. A diagnosis of iron deficiency anemia is made. After 1 month of oral ferrous sulfate 0.3g TID with poor response, the most likely cause is: