Review Question - QID 108667

A 46-year-old woman presents to the emergency department with a 5-month history of fatigue and numbness. She describes a tingling sensation in her fingers and toes and says sometimes she cannot feel them at all. She admits to being an IV heroin user with her last use being yesterday evening. The patient has also been homeless for the last 3 years. She reports that she was sexually assaulted at a shelter so she is afraid to go back. She does not eat regularly but will get meals whenever she can afford them. The patient is currently sexually active with 2 male partners. She has never had testing for sexually transmitted diseases. The patient reports blurry vision but admits she has not been to an eye doctor for over 5 years. She denies fever, headache, palpitations, night sweats, abdominal pain, urinary symptoms, or rashes. Her temperature is 98.6°F (37°C), blood pressure is 114/72 mmHg, pulse is 91/min, and respirations are 12/min. On exam, the patient appears emaciated. Neurologic examination shows a decreased vibratory sense and proprioception in her fingers and toes. Pupils are equally reactive to light and accommodation. The patient has mild difficulty rising from a chair without using her upper extremities to assist. Laboratory results are shown below:

Leukocyte count and differential:
Leukocyte count: 4,400/mm^3
Segmented neutrophils: 60%
Bands: 3%
Eosinophils: 2%
Basophils: < 1%
Lymphocytes: 26%
Monocytes: 7%
Hemoglobin: 9.5 g/dL
Platelet count: 250,000/mm^3
Mean corpuscular hemoglobin concentration: 32%
Mean corpuscular volume: 123 µm^3
Reticulocyte count: .8%

A peripheral blood smear is shown in Figure A. Which of the following is the most likely cause of this patient's symptoms?