A 3-year-old boy presents with recurrent episodes of lethargy, vomiting, and developmental delay. Plasma ammonia is markedly elevated (>200 µmol/L). Genetic testing reveals a deficiency in carbamoyl phosphate synthetase I (CPS I). His 5-year-old sister has similar biochemical findings but remains asymptomatic with normal development. Which feature best distinguishes symptomatic from asymptomatic urea cycle defects in this family?

Explanation

## Discriminating Feature: Hyperammonemia Severity and Residual Enzyme Activity ### Key Pathophysiology **Key Point:** The clinical phenotype of urea cycle defects depends critically on the degree of residual enzyme activity, which determines the severity of hyperammonemia and the rate of ammonia accumulation. In CPS I deficiency, both siblings carry the same genetic mutation but may have different residual enzyme activities due to: - Allelic heterogeneity (different mutations from each parent) - Epigenetic factors - X-inactivation patterns (if X-linked in carrier state) - Metabolic compensation mechanisms ### Why Hyperammonemia Severity Is the Best Discriminator **High-Yield:** The plasma ammonia level is the primary determinant of neurological toxicity in urea cycle defects. Symptomatic patients typically have ammonia >150–200 µmol/L, while asymptomatic individuals may have only mild elevations (50–100 µmol/L) due to greater residual enzyme activity. **Clinical Pearl:** Even within the same genetic defect, patients with >10–20% residual enzyme activity may remain asymptomatic or mildly affected, whereas those with <5% activity develop severe neonatal or early infantile hyperammonemic encephalopathy. ### Comparison Table: CPS I Deficiency Phenotypes | Feature | Symptomatic (Boy) | Asymptomatic (Sister) | | --- | --- | --- | | Plasma ammonia | >200 µmol/L | 50–100 µmol/L | | Residual CPS I activity | <5% | 10–20% | | Plasma citrulline | **Low** (both have low levels) | **Low** (same enzyme defect) | | Urinary orotic acid | Normal or low | Normal or low | | Neurological status | Symptomatic | Asymptomatic | **Warning:** Do NOT confuse CPS I deficiency with OTC deficiency. In OTC deficiency, orotic acid is markedly elevated because carbamoyl phosphate accumulates and is shunted to the pyrimidine synthesis pathway. In CPS I deficiency, carbamoyl phosphate is not produced, so orotic acid is normal or low in both siblings. ### Why Citrulline Levels Are NOT Discriminatory In CPS I deficiency, the enzyme that produces carbamoyl phosphate is defective. Citrulline synthesis requires carbamoyl phosphate + ornithine → citrulline (via ornithine transcarbamylase). Because carbamoyl phosphate is not produced in either sibling, **both will have low plasma citrulline**—this does not distinguish symptomatic from asymptomatic phenotypes. [cite:Robbins 10e Ch 7] [cite:KD Tripathi 8e Ch 12]