Biochemistry Answer-1

(D) A urea cycle disorder (UCD) is likely in a
neonate that has elevated serum ammonia
appearing, not before, but between 24 and 48
hours after a normal term delivery. There are
three hallmark symptoms associated with UCDs.
These are hyperammonemia, encephalopathy,
and respiratory alkalosis. Thus, elevated serum
ammonia is not, in and of itself, indicative of a
specific defect in the urea cycle. An analysis of
the levels of various amino and organic acids in
the plasma and urine is the primary key to determining
which defect led to the elevation in
serum ammonia. Differential diagnosis of
neonatal hyperammonemia, as a consequence of
a UCD can be accomplished by measurement of
plasma citrulline and urinary orotic acid levels.
Shown in Figure 3-6 is the standard differential
diagnosis chart for determining which of four
possible neonatal UCDs is the cause of the
hyperammonemia. First, the hyperammonemia
appears in the absence of any significant acidosis
or ketosis. If analysis of serum citrulline
demonstrates that it is >1000 μM it is confirmation
that the clinical symptoms are due to a deficiency
in ASD. In this circumstance it is not
necessary to assay for levels of urinary orotic
acid but they would be expected to be normal.
Since citrulline levels are dramatically elevated in
cases of ASD, choices A, B, and C are not correct.

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