PR is a 59-year-old woman who presents with respiratory distress and is eventually diagnosed with Streptococcus pneumonia. Her recent medical history includes recent initiation of the antiseizure drug...

PR is a 59-year-old woman who presents with respiratory distress and is eventually diagnosed with Streptococcus pneumonia. Her recent medical history includes recent initiation of the antiseizure drug carbamazepine after a recent stroke. She has a long history of alcohol abuse and is noted by her caseworker to have poor appetite and food intake. Her exam is significant for rhonchi in the right upper lobe. Her labs reveal a pancytopenia with hypersegmented neutrophils. Subsequent bone marrow biopsy reveals a normal-appearing cellular bone marrow. What is the least likely cause of her pancytopenia?

A. Drug reaction

B. Vitamin B12 deficiency

C. Parvovirus B19 infection

D. Alcohol toxicity

E. Pernicious anemia