A 6-month-old male infant presents with developmental delay, hypotonia, and recurrent seizures. Metabolic workup reveals elevated plasma methionine (450 µmol/L, normal <30), elevated homocysteine (85 µmol/L, normal <15), and normal methylmalonic acid. Urine organic acid analysis shows elevated homocysteine metabolites. Vitamin B₁₂ levels are normal. The infant's parents are consanguineous. Which of the following enzyme defects is most likely responsible for this clinical presentation?