A 3-year-old boy from rural Maharashtra is brought to the paediatric emergency department with a 2-day history of progressive lethargy, vomiting, and ataxia. His mother reports he became irritable after a high-protein meal yesterday. On examination, he is drowsy, with a temperature of 37.2°C, heart rate 110/min, and respiratory rate 28/min. There is mild hepatomegaly. Serum ammonia is 380 μmol/L (normal <50), arterial pH 7.32, HCO₃⁻ 16 mEq/L. Plasma amino acid profile 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 Biochemical Clues This child presents with **hyperammonaemia** triggered by dietary protein, manifesting as acute encephalopathy (lethargy, ataxia, vomiting). The key diagnostic finding is **markedly elevated urinary orotic acid** in the context of hyperammonaemia. ## Urea Cycle Enzyme Deficiencies: Differential | Enzyme Defect | Ammonia | Orotic Acid | Citrulline | Arginine | Key Feature | |---|---|---|---|---|---| | **OTC deficiency** | ↑↑ | ↑↑↑ (markedly) | ↓ | ↓ | **Most common; X-linked; females symptomatic** | | CPS I deficiency | ↑↑ | Normal/low | ↓ | ↓ | Rare; no orotic acid elevation | | Argininosuccinate lyase | ↑ | Normal | ↑↑ | ↑ | Argininosuccinate in urine | | Argininosuccinate synthetase | ↑ | Normal | ↑↑ | ↓ | Citrulline in urine | ## Why OTC Deficiency Explains This Case **Key Point:** OTC deficiency is the **most common urea cycle disorder** (1 in 14,000 births). It is **X-linked**, so males are typically more severely affected, though heterozygous females can be symptomatic (as in this case, if female, or if male hemizygous). **Biochemical mechanism:** - OTC catalyzes: Carbamoyl phosphate + Ornithine → Citrulline - When OTC is deficient, carbamoyl phosphate accumulates - Excess carbamoyl phosphate is shunted into the **pyrimidine synthesis pathway** (carbamoyl phosphate → orotate → orotic acid) - Result: **markedly elevated urinary orotic acid** (pathognomonic for OTC deficiency) - Ammonia cannot be incorporated into the urea cycle → **hyperammonaemia** **Clinical Pearl:** The **elevated urinary orotic acid is the diagnostic hallmark** that distinguishes OTC deficiency from CPS I deficiency (both present with hyperammonaemia and low citrulline, but only OTC causes orotic acid elevation). **High-Yield:** OTC deficiency is **X-linked recessive**; heterozygous females may present with hyperammonaemia after protein loading or illness. Males are usually more severely affected in infancy. **Mnemonic:** **OTC = Orotic acid Tremendously elevated in urine, Carbamoyl phosphate accumulates** ## Management Approach ```mermaid flowchart TD A[Hyperammonaemia + Elevated Orotic Acid]:::outcome --> B{OTC Deficiency Suspected?}:::decision B -->|Yes| C[Restrict protein intake]:::action C --> D[Sodium benzoate or phenylbutyrate]:::action D --> E[Scavenge ammonia via alternate pathway]:::outcome B -->|Confirm| F[Plasma amino acids, urine orotic acid]:::action F --> G[Genetic testing: OTC gene]:::action ``` **Treatment:** Protein restriction, ammonia scavengers (sodium benzoate, sodium phenylbutyrate), and arginine supplementation to promote urea cycle flux. [cite:Robbins 10e Ch 7]