you suspect that a 14 year old male patient has an inborn error of methionine metabolism. you order several tests and find that his homocysteine levels are elevated, but methionine levels are low. a liver biopsy demonstrates cystathionine synthase and cystathionase levels are normal. this patient may have a deficiency of
a. serine
b. threonine
c. vitamin B12
d. methylmalonyl CoA
e. succinyl CoA