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A 14-year-old boy is brought to the pediatrician by his mother over concerns of difficulty walking and worsening clumsiness. He reports a gradual loss of night vision, and a long history of chronic diarrhea which is pale and foul-smelling. Examination reveals an ataxic gait, and loss of deep tendon reflexes. Bilateral retinitis pigmentosa is noted on fundoscopy and a peripheral blood smear reveals the findings seen here. Which additional laboratory finding is most likely? Image credit: @bkehmsen.