Homocystinuria Marfanoid Downward Lens

A 14-year-old boy presents with tall stature, learning difficulties, and a history of eye surgery at age 9 for lens dislocation. On slit-lamp examination, the structure marked **A** (downward displacement of the crystalline lens) is noted in the right eye, with high myopia bilaterally. Plasma homocysteine is markedly elevated at 240 micromol/L (normal <15), and methionine is elevated at 320 micromol/L. The cyanide-nitroprusside test on urine is strongly positive. Parents are first cousins. Which of the following best explains the ocular finding marked **A** in this clinical context?