A 10-yeаr-оld child is evаluаted fоr develоpmental delays, nearsightedness, and recurrent blood clots. Laboratory testing reveals markedly elevated homocysteine levels in blood and urine. The physician suspects a form of homocystinuria caused by impaired conversion of homocysteine to downstream sulfur-containing metabolites. Which vitamin is most likely to improve symptoms in some forms of this disorder?