Review Question - QID 108699

A 54-year-old female presents to her primary care provider for a 3 week history of fatigue. She reports that she has noticed that she is more tired after any activity and feels weaker at the end of the day. She denies any changes to her diet and she reports that she still manages to get nearly 8 hours of sleep per night. She reports recent travel to Norway where she ate uncooked freshwater fish for many of her meals. She has recently also had several bouts of severe abdominal pain and bloody diarrhea that required a trip to the emergency department. Her past medical history is significant for hypertension, hyperlipidemia, diabetes mellitus, epilepsy, Crohn disease, and gastroesophageal reflux disease. Her home medications include hydrochlorothiazide, lisinopril, rosuvastatin, metformin, phenytoin, ranitidine, and omeprazole. Her temperature is 98.8°F (37.1°C), blood pressure is 143/94 mmHg, pulse is 73/min, and respirations are 14/min. On physical exam, she is noted to have conjunctival pallor with a normal neurological exam. Laboratory tests are performed with the following results:

Leukocyte count: 5,600/mm^3
Hemoglobin: 9.7 g/dL
Hematocrit: 31%
Mean corpuscular volume: 108 µm^3
Platelet count: 246,000/mm^3

Her serum methylmalonic acid level is within normal limits. A peripheral blood smear is obtained and the results are shown in Figure A. Which of the following is the most likely cause of this patient’s symptoms?