A 3-day-old male neonate born to consanguineous parents presents with poor feeding, lethargy, and seizures. Serum ammonia is 280 µmol/L (normal <50). Plasma amino acid profile shows elevated glutamine and alanine with low citrulline. Which investigation is most appropriate to confirm the specific urea cycle enzyme defect?

Explanation

## Diagnostic Approach to Urea Cycle Defects ### Clinical Presentation Recognition The neonatal presentation with hyperammonemia, elevated glutamine/alanine, and **low citrulline** is pathognomonic for **Carbamoyl Phosphate Synthetase I (CPS I) deficiency** or **Ornithine Transcarbamylase (OTC) deficiency**. The low citrulline is the key discriminator. ### Why Urine Orotic Acid is the Gold Standard **Key Point:** Urine orotic acid quantification differentiates between CPS I and OTC deficiency: - **OTC deficiency** → **markedly elevated urine orotic acid** (100–1000× normal) because carbamoyl phosphate accumulates and is shunted to pyrimidine synthesis - **CPS I deficiency** → **normal or low urine orotic acid** because carbamoyl phosphate synthesis is blocked upstream HPLC (High-Performance Liquid Chromatography) is the most specific and quantitative method for orotic acid measurement, making it the **investigation of choice** for distinguishing these two defects. ### Clinical Pearl **High-Yield:** In a hyperammonemic neonate with low citrulline: 1. Check **urine orotic acid first** (non-invasive, rapid, diagnostic) 2. If elevated → OTC deficiency (X-linked, more common) 3. If normal → CPS I deficiency (autosomal recessive, rarer) ### Why This Matters - OTC deficiency is **X-linked recessive** (males severely affected) - CPS I deficiency is **autosomal recessive** (both sexes equally affected) - Genetic counseling and prenatal diagnosis differ ## Diagnostic Algorithm ```mermaid flowchart TD A[Neonatal hyperammonemia + low citrulline]:::outcome --> B[Check urine orotic acid]:::action B --> C{Orotic acid level?}:::decision C -->|Markedly elevated| D[OTC deficiency]:::outcome C -->|Normal/low| E[CPS I deficiency]:::outcome D --> F[X-linked inheritance, males severe]:::outcome E --> G[Autosomal recessive, both sexes]:::outcome ``` [cite:Robbins 10e Ch 7]