Biochemistry-1

A deficiency of argininosuccinate synthetase
(ASD) can be suspected in a 24–48-hour-old
neonate with elevated serum ammonia. To discriminate
the fact that the hyperammonemia is
indeed due to ASD and not due to a deficiency
in other urea cycle enzymes, one can assay for
the serum concentration of citrulline and urinary
orotic acid concentration. Which of the
following would best describe the expected
findings?
(A) Both citrulline and orotate levels will be
low.
(B) Citrulline levels will be absent but orotate
will be elevated.
(C) Citrulline levels will be between
100–300 μM and orotate levels will be
elevated.
(D) Citrulline levels will be >1000 μM, orotate
will be near normal.

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