Vitamin B12 deficiency Quiz: Ace Your Exams

A 32-year-old woman comes to her primary care physician with worsening depression and frequent falls. She has had difficulty sleeping at night and has frequently been stumbling over herself while walking. The patient states “my legs feel numb all the time.” Her partner, who is also present during the visit, states that the patient has not been herself lately and has been eating less since her mother passed away. Past medical history is notable for Crohn disease, depression, and Graves disease. She drinks alcohol socially, occasionally smokes marijuana, and adheres to a vegan diet. Her temperature is 37.0°C (98.6°F), pulse is 67/min, respirations are 14/min, blood pressure is 110/74 mmHg, and O2 saturation is 98% on room air. Physical examination is notable for a sad affect, an ataxic gait, as well as paresthesias and hyperactive deep tendon reflexes in the lower extremities bilaterally. Laboratory testing reveals the following results:  

Laboratory Value	Result
Hemoglobin	10.5 g/dL
Leukocyte count	1,000/mm3
Platelet count	90,000/mm3
Mean corpuscular volume	115 μm3

Which of the following is the most likely etiology of this patient’s symptoms?