55-year-old wo

man

with a history of chronic kidney disease comes

to the physician for a 2-month history of easy

fatigability. Physical examination shows subconjunctival pallor. Her hemoglobin

concentration is 80 g/l. Treatment with erythropoietin has been administered. A

peripheral blood smear is obtained one week after treatment. A photomicrograph

of the smear after specialized staining is shown. The prominent color of the intracellular structure in

some of the cells is most likely the result of staining which of the following?