Review Question - QID 214677

A 42-year-old woman presents to the emergency department with complaints of difficulty breathing for the past 2 weeks. She reports shortness of breath whenever she goes on her morning run. The symptoms last for approximately 5-10 minutes and improve with rest. She denies chest pain, syncope, nausea, or abdominal pain during these episodes. Her medical history is significant for rheumatoid arthritis which is treated with hydroxychloroquine. She recently returned from a business trip to Japan. She denies any recent surgeries, oral contraceptive/estrogen use, malignancy, or personal history of deep vein thrombosis (DVT) or pulmonary embolism (PE). She endorses some rhinorrhea and sore throat that has since resolved. The patient admits to having dark stools over the past few days but denies any frank blood. Physical examination demonstrates some joint swelling at the proximal interphalangeal joints bilaterally but is otherwise unremarkable. Laboratory studies are shown below, and a chest radiograph is shown in Figure A.

Leukocyte count and differential: 9,800/mm^3 with normal differential
Hemoglobin: 8 g/dL
Platelet count: 180,000/mm^3
Mean corpuscular hemoglobin concentration: 32%
Mean corpuscular volume (MCV): 83 µm^3
Reticulocyte count: 0.2%
Ferritin: 268 ng/mL (Normal: 12-150 ng/mL)
Serum iron: 38 mcg/dL (Normal: 60-170 mcg/dL)
Total iron binding capacity (TIBC): 240 mcg/dL (Normal: 240-450 mcg/dL)

What is the most likely explanation for this patient’s symptoms?