Question:medium
Which amino acid needs to be supplemented through diet in a patient with cystathionine beta synthase deficiency?
Which amino acid needs to be supplemented through diet in a patient with cystathionine beta synthase deficiency?
Show Hint
Think about which product of the transsulfuration pathway cannot be formed when CBS is absent.
Updated On: Jun 23, 2026
- Cysteine
- Methionine
- Serine
- Tryptophan
Show Solution
The Correct Option is A
Solution and Explanation
In the transsulfuration pathway: Homocysteine + Serine $\xrightarrow{\text{CBS, PLP}}$ Cystathionine $\xrightarrow{\text{Cystathionase, PLP}}$ Cysteine + Homoserine.
CBS deficiency blocks the first step, so cystathionine is not formed and consequently cysteine cannot be synthesized endogenously.
Key memory point: Cysteine is a non-essential amino acid in normal physiology, but becomes essential (must come from diet) in homocystinuria due to CBS deficiency.
Other options: Methionine is the upstream precursor -- it accumulates rather than being deficient. Serine is a substrate but is not depleted to a critical level. Tryptophan is unrelated to this pathway.
\[\boxed{\text{Cysteine}}\]
CBS deficiency blocks the first step, so cystathionine is not formed and consequently cysteine cannot be synthesized endogenously.
Key memory point: Cysteine is a non-essential amino acid in normal physiology, but becomes essential (must come from diet) in homocystinuria due to CBS deficiency.
Other options: Methionine is the upstream precursor -- it accumulates rather than being deficient. Serine is a substrate but is not depleted to a critical level. Tryptophan is unrelated to this pathway.
\[\boxed{\text{Cysteine}}\]
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