A 3-day-old male neonate born to non-consanguineous parents presents with poor feeding, lethargy, and vomiting. On examination, he is hypotonic with a weak cry. Laboratory investigations reveal: blood ammonia 180 µmol/L (normal <50), plasma glutamine 1200 µmol/L (normal 400–700), and normal blood glucose and liver function tests. Urine orotic acid is markedly elevated. Plasma citrulline is low. Which enzyme deficiency is most likely responsible for this presentation?

Explanation

## Clinical Presentation & Biochemistry This neonate presents with the classic triad of **neonatal hyperammonemia**: poor feeding, lethargy, and vomiting within the first few days of life, accompanied by severe hyperammonemia (180 µmol/L) and elevated glutamine. ## Diagnostic Clues **Key Point:** The combination of **elevated orotic acid in urine** and **low plasma citrulline** is pathognomonic for OTC deficiency. ### Why OTC Deficiency? 1. **Orotic Acid Elevation**: OTC catalyzes the second step of the urea cycle (carbamoyl phosphate + ornithine → citrulline). When OTC is deficient: - Carbamoyl phosphate accumulates - Carbamoyl phosphate is shunted into the pyrimidine synthesis pathway → **massive orotic acid production** → orotic aciduria 2. **Low Citrulline**: Citrulline cannot be formed (OTC is the enzyme that produces it), so plasma citrulline is reduced. 3. **Hyperammonemia**: Carbamoyl phosphate cannot enter the urea cycle → ammonia accumulates. 4. **X-Linked Inheritance**: OTC is X-linked; males are more severely affected than heterozygous females. ## Urea Cycle Enzyme Deficiencies — Differential | Enzyme | Orotic Acid | Citrulline | Ammonia | Inheritance | |--------|-------------|-----------|---------|-------------| | **OTC** | ↑↑↑ (marked) | ↓ | ↑↑↑ | X-linked | | **CPS I** | Normal | ↓ | ↑↑↑ | Autosomal | | **Argininosuccinate synthetase** | Normal | ↑↑ | ↑ | Autosomal | | **Argininosuccinate lyase** | Normal | ↑↑ | ↑ | Autosomal | **High-Yield:** Orotic aciduria is **unique to OTC deficiency** among urea cycle disorders. ## Pathophysiology Diagram ```mermaid flowchart TD A[Ammonia + Glutamine] --> B[Carbamoyl Phosphate Synthetase I] B --> C[Carbamoyl Phosphate] C -->|OTC enzyme| D[Citrulline] D --> E[Argininosuccinate] E --> F[Arginine] F --> G[Urea + Ornithine] C -->|OTC deficiency| H[Shunt to Pyrimidine Pathway]:::urgent H --> I[Orotic Acid]:::urgent H --> J[Orotic Aciduria]:::urgent D -.->|Cannot form| K[Low Plasma Citrulline]:::outcome C -.->|Accumulates| L[Hyperammonemia]:::urgent ``` **Clinical Pearl:** OTC deficiency is the **most common urea cycle disorder** (~50% of all cases) and the only X-linked form, making it more common in males with severe neonatal presentation.