A 16-year-old boy from rural Maharashtra presents with acute ischemic stroke. On examination, he is tall with long limbs, arachnodactyly, and bilateral downward lens dislocation. Serum homocysteine is markedly elevated (>100 μmol/L) and urine shows homocysteine crystals. The structure marked **B** in the diagram is deficient due to an autosomal recessive mutation. Which of the following best explains the pathophysiology of his thromboembolic presentation?