Classical homocystinuria is caused by a deficiency of which enzyme?

• A. Cystathionine beta-synthase deficiency
• B. Methionine synthase deficiency
• C. Cystathione synthase deficiency
• D. Homocysteine methyltransferase deficiency

Answer: (C)

Classic homocystinuria results from a deficiency of cystathionine beta-synthase, the enzyme in the transsulfuration pathway that converts homocysteine and serine into cystathionine. When this step is impaired, homocysteine builds up and is diverted toward methionine, so both homocysteine and methionine rise while cysteine falls. This biochemical pattern underlies the typical clinical signs such as lens subluxation, marfanoid habitus, thromboembolism, and developmental issues. The correct enzyme name to remember is cystathionine beta-synthase. Other options refer to remethylation steps (which cause a different form of homocystinuria) or are misspelled, so they do not explain classic homocystinuria.