A 6-month-old male infant presents with poor feeding, lethargy, and vomiting after starting protein-rich feeds. Plasma ammonia is 180 µmol/L (normal <50). Genetic testing reveals a deficiency in carbamoyl phosphate synthetase I (CPS I). Which clinical and biochemical feature best distinguishes CPS I deficiency from ornithine transcarbamylase (OTC) deficiency?

## Distinguishing CPS I from OTC Deficiency ### The Urea Cycle Checkpoint **Key Point:** CPS I and OTC catalyze the first two committed steps of the urea cycle. Their deficiencies produce distinct biochemical signatures based on substrate accumulation and product depletion. ### Biochemical Mechanism In **CPS I deficiency**: - Carbamoyl phosphate cannot be synthesized from ammonia + CO₂ + glutamine - Ammonia accumulates upstream (hyperammonemia) - Carbamoyl phosphate is depleted → no orotic acid synthesis from the salvage pathway - Plasma citrulline is **low** (no carbamoyl phosphate to combine with ornithine) In **OTC deficiency**: - Carbamoyl phosphate accumulates (cannot be used) - Carbamoyl phosphate shunts into pyrimidine synthesis → **elevated orotic acid** (urine) - Ammonia accumulates (cannot enter the cycle) - Plasma citrulline is **low** (substrate not converted to citrulline) ### Comparison Table | Feature | CPS I Deficiency | OTC Deficiency | |---------|------------------|----------------| | **Plasma ammonia** | ↑↑ (severe) | ↑↑ (severe) | | **Urinary orotic acid** | Normal/low | ↑↑↑ (markedly elevated) | | **Plasma citrulline** | ↓ (low) | ↓ (low) | | **Carbamoyl phosphate** | ↓ (depleted) | ↑ (accumulated) | | **Sex-linked inheritance** | Autosomal recessive | X-linked dominant (lethal in males) | ### Why Orotic Acid is the Discriminator **High-Yield:** Orotic acid is the **pathognomonic finding** of OTC deficiency. It accumulates because: 1. Carbamoyl phosphate cannot exit the urea cycle (blocked at OTC step) 2. It enters the pyrimidine synthesis pathway (carbamoyl phosphate → carbamoyl aspartate → orotic acid) 3. Orotic acid is excreted in large amounts in urine In CPS I deficiency, carbamoyl phosphate is depleted, so there is **no substrate** for orotic acid synthesis — urinary orotic acid remains normal. **Clinical Pearl:** A neonate with hyperammonemia and **crystalline orotic acid in urine** (or elevated urinary orotic acid on organic acid analysis) is OTC deficiency until proven otherwise. This finding is absent in CPS I deficiency. ### Mnemonic **OROTIC = OTC Regulatory Overflow** — Orotic acid accumulates and spills into urine only when OTC is blocked. [cite:Robbins 10e Ch 7] [cite:KD Tripathi 8e Ch 12]