Final answer:
In stage 3 iron deficiency, free erythrocyte protoporphyrin (FEP) is increased as hemoglobin synthesis is impaired, leading to FEP accumulation in red blood cells. Serum transferrin receptors are also increased to enhance iron uptake.
Step-by-step explanation:
Among the options provided for the characteristics of stage 3 iron deficiency, the correct statement is that free erythrocyte protoporphyrin (FEP) is increased. This is because when there is insufficient iron, hemoglobin synthesis is disrupted, which leads to an accumulation of protoporphyrin in red blood cells. On the other hand, serum transferrin receptors are increased because the body tries to increase iron uptake due to deficiency. The number of sideroblasts may not necessarily be normal; it could be decreased or show abnormal iron accumulation. Lastly, in iron-deficiency anemia the red cell count is decreased, and the hematocrit is also decreased due to the smaller number and volume of red blood cells.