A 3-day-old male neonate born to non-consanguineous parents presents with poor feeding, lethargy, and seizures. The child was born after an uncomplicated pregnancy and delivery with normal Apgar scores. On examination, the infant is hypotonic with a weak cry. Laboratory investigations reveal: blood ammonia 380 µmol/L (normal <50), blood glucose 65 mg/dL, arterial pH 7.32 with respiratory alkalosis, and normal liver function tests. Plasma amino acid analysis shows elevated glutamine and alanine. Urine orotic acid is markedly elevated. Which enzyme deficiency is most likely responsible for this presentation?

Explanation

## Clinical Presentation and Diagnosis This neonatal case presents with the classic triad of **hyperammonemia, encephalopathy, and respiratory alkalosis** in the first week of life—hallmark of acute urea cycle defect (UCD). ### Key Diagnostic Clue: Elevated Urinary Orotic Acid **Key Point:** Markedly elevated urinary orotic acid is pathognomonic for **OTC deficiency**. This occurs because: 1. Carbamoyl phosphate accumulates when OTC cannot convert it to citrulline 2. Excess carbamoyl phosphate shunts into the pyrimidine synthesis pathway 3. This produces excessive orotic acid, which is excreted in urine No other urea cycle enzyme deficiency produces this degree of orotic acid elevation. ### Why OTC Deficiency Fits This Case | Feature | OTC Deficiency | CPS I Deficiency | Argininosuccinate Lyase | Argininosuccinate Synthetase | |---------|---|---|---|---| | **Urinary orotic acid** | ↑↑↑ (markedly elevated) | Normal or low | Normal | Normal | | **Plasma ammonia** | Very high (>300) | Very high | Moderate | Moderate | | **Age of onset** | Neonatal (most common UCD) | Neonatal | Neonatal/infantile | Neonatal/infantile | | **Inheritance** | X-linked (males > females) | Autosomal recessive | Autosomal recessive | Autosomal recessive | | **Plasma amino acids** | ↑ Glutamine, ↑ Alanine | ↑ Glutamine, ↑ Alanine | ↑ Argininosuccinate | ↑ Argininosuccinate | **High-Yield:** OTC deficiency accounts for ~50% of all UCDs and is the most common cause of neonatal hyperammonemia in developed countries. ### Pathophysiology ```mermaid flowchart TD A[Protein/Amino acid catabolism]:::action --> B[Ammonia produced]:::action B --> C[Carbamoyl phosphate synthetase I]:::action C --> D[Carbamoyl phosphate formed]:::action D --> E{OTC enzyme present?}:::decision E -->|No - OTC deficiency| F[Carbamoyl phosphate accumulates]:::urgent E -->|Yes| G[Citrulline formed]:::action F --> H[Shunts to pyrimidine synthesis]:::action H --> I[Excessive orotic acid production]:::urgent I --> J[Marked urinary orotic acid]:::outcome F --> K[Ammonia accumulates]:::urgent K --> L[Hyperammonemia + encephalopathy]:::urgent ``` **Clinical Pearl:** The respiratory alkalosis (pH 7.32 with low PaCO₂) occurs because ammonia directly stimulates the respiratory center in the medulla, causing hyperventilation and CO₂ loss. ### Management Implications **Key Point:** OTC deficiency is treated with: - Protein restriction - Nitrogen scavenging agents (sodium benzoate, sodium phenylbutyrate) - Arginine supplementation (bypasses the block) - Hemodialysis for acute hyperammonemic crisis [cite:Robbins 10e Ch 7]