A 3-year-old boy presents with developmental delay, seizures, and hyperammonemia (plasma ammonia 180 µmol/L). Genetic testing reveals a deficiency in carbamoyl phosphate synthetase I (CPS I). His 6-year-old sister has similar symptoms but normal CPS I activity; genetic testing shows ornithine transcarbamylase (OTC) deficiency. Which clinical feature best distinguishes OTC deficiency from CPS I deficiency?

Explanation

## Distinguishing OTC Deficiency from CPS I Deficiency ### Key Biochemical Difference **Key Point:** Elevated urinary orotic acid is the pathognomonic finding that distinguishes OTC deficiency from CPS I deficiency. ### Mechanism In **OTC deficiency**, carbamoyl phosphate (produced normally by CPS I) cannot be converted to citrulline and accumulates. Excess carbamoyl phosphate is shunted into the pyrimidine synthesis pathway, leading to: 1. Increased carbamoyl phosphate → orotate production 2. Massive urinary excretion of orotic acid (>1000× normal) In **CPS I deficiency**, carbamoyl phosphate is not produced, so the pyrimidine pathway is not activated — urinary orotic acid remains normal. ### Comparison Table | Feature | OTC Deficiency | CPS I Deficiency | | --- | --- | --- | | **Urinary orotic acid** | ↑↑↑ (markedly elevated) | Normal | | **Plasma citrulline** | Low/absent | Low/absent | | **Plasma ammonia** | ↑↑↑ | ↑↑↑ | | **Inheritance** | X-linked (males > females) | Autosomal recessive (equal) | | **Neonatal presentation** | Males (hemizygous) | Both sexes equally | ### Clinical Pearl **Clinical Pearl:** Orotic acid crystalluria (orange crystals in urine) is a classic sign of OTC deficiency and may be visible on urinalysis — a simple bedside clue. ### High-Yield Mnemonic **Mnemonic:** **OTC = Orotic acid Tremendously elevated in Carbamoyl phosphate accumulation** When carbamoyl phosphate synthase I works but the next step (OTC) fails, carbamoyl phosphate backs up into pyrimidine synthesis → orotic acid overproduction. [cite:Lehninger Principles of Biochemistry Ch 23]