A 35-year-old woman underwent partial gastrectomy 10 years ago due to gastric bleeding. Over the past year, she has complained of dizziness, fatigue, and pallor. Laboratory tests show: RBC 3.2×10¹²/L, Hb 70g/L, normal WBC and PLT counts, reticulocytes 0.015 (5%). Bone marrow examination reveals marked erythroid hyperplasia, predominantly intermediate and late normoblasts. These cells are small with scant basophilic cytoplasm and irregular margins. Which of the following is most likely observed in serum iron studies?