Citrullinemia Type I (argininosuccinate synthetase deficiency) differs from ornithine transcarbamylase (OTC) deficiency in which key biochemical marker?

• A Orotic acid is absent in citrullinemia; elevated in OTC deficiency
• B Citrulline is markedly elevated in citrullinemia; low/absent in OTC deficiency ✓
• C Both conditions show identical plasma amino acid patterns
• D Argininosuccinate is elevated in citrullinemia Type I

Explanation

In OTC deficiency, the block is before citrulline formation, so plasma citrulline is very low and carbamoyl phosphate overflows the mitochondria into the cytosol, driving pyrimidine synthesis and elevating urinary orotic acid. In citrullinemia Type I, the block is after citrulline formation (argininosuccinate synthetase converts citrulline + aspartate → argininosuccinate), so plasma citrulline markedly accumulates while orotic acid may be mildly elevated. Argininosuccinate accumulates in argininosuccinate lyase deficiency (citrullinemia Type II involves citrin deficiency).

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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