Review Question - QID 215146

A 26-year-old man presents for evaluation of decreased exercise capacity. He states that he recently started participating in a jogging club but has had difficulty keeping up with others despite many weeks of training. He was born to Greek parents, is vegan, and works at a fishing lure company where he makes weights out of lead. His temperature is 98.1°F (36.7°C), blood pressure is 118/68 mmHg, pulse is 88/min, and respirations are 15/min. On exam, he is slightly pale appearing. Laboratory evaluation reveals:

Leukocyte count: 7,500/mm^3
Hemoglobin: 10.1 g/dL
Erythrocyte count: 5.8 million/mm^3
Platelet count: 200,000/mm^3
Mean corpuscular volume (MCV): 62 µm^3
Red cell distribution width (RDW): 12% (normal: 12-15%)
Serum iron: 95 mcg/dL (normal: 80-180 mcg/dL)
Ferritin: 150 ng/mL

A peripheral blood smear is performed, as shown in Figure A. Which of the following is the most likely etiology of this patient’s anemia?