Urea cycle: carbamoyl phosphate synthetase I
Which enzyme catalyzes the rate-limiting step of the urea cycle?
Answer B: explanation for every choice
A
Arginase catalyzes the final step, releasing urea.
B
Correct. CPS I in the mitochondria is rate-limiting and is activated by N-acetylglutamate.
C
Ornithine transcarbamylase deficiency is the most common urea cycle disorder, but it is not the rate-limiting step.
D
CPS II is cytosolic and begins pyrimidine synthesis.
TakeawayCPS I (mitochondria) → urea cycle. CPS II (cytosol) → pyrimidines.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb68.