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 plasma ammonia 280 µmol/L (normal <50), normal blood glucose, normal liver function tests, and normal plasma amino acid profile except for elevated glutamine. Urine orotic acid is markedly elevated. Cerebrospinal fluid shows no evidence of infection. What is the most likely diagnosis?

Explanation

## Clinical Presentation and Key Findings This neonatal presentation with hyperammonemia, elevated glutamine, and **markedly elevated urinary orotic acid** is pathognomonic for **OTC deficiency**. ### Why Elevated Orotic Acid Occurs in OTC Deficiency **Key Point:** OTC catalyzes the condensation of carbamoyl phosphate and ornithine to form citrulline. When OTC is deficient, carbamoyl phosphate accumulates and is shunted into the pyrimidine synthesis pathway, leading to excessive orotic acid production and urinary excretion. ### Biochemical Mechanism ```mermaid flowchart TD A[Carbamoyl Phosphate]:::outcome --> B{OTC present?}:::decision B -->|Yes| C[Citrulline formation]:::action C --> D[Urea cycle proceeds]:::action B -->|No| E[Carbamoyl phosphate accumulates]:::urgent E --> F[Shunted to pyrimidine synthesis]:::action F --> G[Orotic acid ↑↑↑]:::outcome G --> H[Urinary orotic acid elevation]:::outcome ``` ### Diagnostic Features of OTC Deficiency | Feature | OTC Deficiency | CPS I | NAGS | Argininosuccinate Lyase | |---------|---|---|---|---| | **Plasma ammonia** | ↑↑↑ | ↑↑↑ | ↑↑ | ↑↑ | | **Plasma glutamine** | ↑↑ | ↑↑ | ↑ | ↑ | | **Urinary orotic acid** | **↑↑↑ (marked)** | Normal | Normal | Normal | | **Inheritance** | X-linked | Autosomal recessive | Autosomal recessive | Autosomal recessive | | **Age of onset** | Neonatal (males) | Neonatal | Neonatal | Neonatal/infantile | | **Citrulline level** | Low | Low | Low | ↑↑ | **High-Yield:** The combination of **hyperammonemia + elevated urinary orotic acid** is virtually diagnostic of OTC deficiency. No other urea cycle defect produces this pattern. ### Clinical Pearl **Clinical Pearl:** OTC deficiency is the most common urea cycle disorder (accounts for ~50% of cases) and is X-linked dominant. Hemizygous males present in the neonatal period with severe hyperammonemia; heterozygous females may present later with milder symptoms triggered by infection, high protein intake, or pregnancy. ### Management Considerations 1. **Immediate:** Reduce ammonia via protein restriction, nitrogen-scavenging agents (sodium benzoate, sodium phenylbutyrate), and arginine supplementation 2. **Chronic:** Protein-restricted diet, ammonia-lowering medications, genetic counseling 3. **Prognosis:** Early diagnosis and treatment can prevent neurological damage; delayed treatment leads to hyperammonemic encephalopathy and death [cite:Robbins 10e Ch 7]