Review Question - QID 213771

A 31-year-old woman presents to her primary care physician with a 4 month history of fatigue and shortness of breath. She says that she started experiencing these symptoms while exercising but assumed that was because she was starting to get older. Over time, the symptoms have gotten worse such that she now becomes fatigued even while doing activities of daily living like shopping. Her past medical history is significant for rheumatoid arthritis for which she takes methotrexate. She is vegan, drinks 2 glasses of wine per night, and lives in an old house. Family history reveals multiple relatives with anemia. Physical exam reveals a thin, disheveled woman with conjunctival pallor. Selected laboratory tests are shown below:

Hemoglobin: 9.5 g/dL
Mean corpuscular volume: 73 fl
Ferritin: 230 ng/ml

Which of the following is most likely responsible for this patient's symptoms?