During a Routine Examination of a 58-Year-Old Male Patient, Blood

During a routine examination of a 58-year-old male patient, blood work indicates a significant elevation in the level of homocysteine. Elevated levels of homocysteine in the blood have been shown to correlate with cardiovascular dysfunction and thus, it is imperative to determine the cause of this patient's homocysteine levels. Analysis for activity of which of the following enzymes is likely to show a defect resulting in the elevation in serum homocysteine in this patient?

A) cystathionine β-synthase
B) dihydrofolate reductase
C) dihydropteridine reductase
D) methionine adenosyltransferase
E) methionine synthase