A 25-year-old 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, and increased central pallor of RBCs on smear. Iron deficiency anemia is suspected. The most likely etiology is: