Review Question - QID 107987

A 45-year-old female patient presents to her primary care physician 4 years after gastric bypass surgery complaining of numbness and tingling in her feet and fingertips. She has also been feeling fatigued over the last few months and cannot walk far before becoming winded. Physical exam shows conjunctival pallor and pale skin. Lab tests are conducted and demonstrate a hematocrit of 31% (normal range 35%-45%) and Mean Corpuscular Volume (MCV) of 112 fl (normal range 80 fl - 100 fl). A peripheral blood smear is obtained and is shown in Figure A. What is the most likely cause of this patient's clinical syndrome?