A 25-year-old male farmer presents with pallor and fatigue for one year. Blood tests show RBC: 2.0×10<sup>9</sup>/L, Hb: 50g/L, WBC: 7.6×10<sup>9</sup>/L, neutrophils: 0.50, lymphocytes: 0.26, eosinophils: 0.14, serum ferritin: 10μg/L, and increased central pallor of red blood cells on peripheral smear. A diagnosis of iron-deficiency anemia is made. The patient is treated with ferrous sulfate 0.3g three times daily orally for one month but shows poor response. The most likely reason is: