Review Question - QID 214191

A 55-year-old woman presents to clinic with fatigue and decreased exercise tolerance. Over the past year, she has found it increasingly difficult to walk to work because she gets short of breath. She also feels tired after climbing 3 flights of stairs, which has never been a problem in the past. She denies orthopnea, cough, weight loss, or fevers. She has a history of rheumatoid arthritis and hypertension. She had an aortic valve replacement 5 years ago. She is taking no medications and has no allergies. She smoked 1 pack per day for 10 years but recently quit and started a vegan diet 2 weeks ago. Her temperature is 99.1°F (37.3°C), blood pressure is 150/85 mmHg, pulse is 65/min, and respirations are 15/min. On exam, she has mucosal pallor and decreased capillary refill. She has a regular heart rate with a third heart sound before S1. There is bilateral swelling with ulnar deviation of the metacarpophalangeal joints and bilateral swelling of her knees. The remainder of the exam is unremarkable. Routine labs are shown below.

Hemoglobin: 8.0 g/dL
Mean corpuscular volume: 81 µm^3
Serum iron: 35 µg/dL
Total iron binding capacity: 150 mcg/dL
Ferritin: 500 ng/mL
LDH: 200 U/L
Haptoglobin: 100 mg/dL

Which of the following is the most likely cause of this patient's anemia?