A 34-year-old primigravida is in her 8th month of gestation. She is feeling increasingly tired and weak. Laboratory studies include a CBC which shows Hgb 9.7 g/dL, Hct 28.8%, MCV 71 fL, platelet count 289,000/microliter, and WBC count 5600/microliter. On the peripheral blood smear, the RBCs show increased variation in size and shape, with many that are hypochromic and microcytic. Which of the following dietary deficiencies is she most likely to have?