A 6-month-old female infant from a consanguineous family presents with recurrent vomiting, poor weight gain, and developmental delay. She has a history of two previous siblings who died in infancy with similar presentations. Current investigations show: blood ammonia 180 µmol/L, plasma arginine 8.5 µmol/L (normal 40–130), plasma citrulline 42 µmol/L (normal 15–45), and urine orotic acid is normal. Liver function tests and blood glucose are normal. Which enzyme deficiency best explains this clinical picture?

Explanation

## Clinical and Biochemical Analysis This case presents a classic picture of **argininosuccinate synthetase (ASS) deficiency**, also known as **citrullinemia type I**. ### Key Diagnostic Features **Key Point:** The pathognomonic finding is **markedly elevated plasma citrulline with low arginine**: - Plasma citrulline: 42 µmol/L (at upper limit of normal, but relatively high in context of low arginine) - Plasma arginine: 8.5 µmol/L (severely low, normal >40) - Hyperammonemia: moderate (180 µmol/L) - **Normal urine orotic acid** (distinguishes from OTC deficiency) ### Urea Cycle Defect Comparison | Enzyme Defect | Plasma Citrulline | Plasma Arginine | Plasma Argininosuccinate | Urine Orotic Acid | Severity | | --- | --- | --- | --- | --- | --- | | **ASS deficiency (Citrullinemia I)** | ↑↑↑ (markedly elevated) | ↓↓ (very low) | Normal | Normal | Moderate–severe | | **Argininosuccinate lyase** | ↑ (elevated) | ↑ (elevated) | ↑↑↑ (markedly elevated) | ↑ (elevated) | Moderate | | **OTC deficiency** | ↓ (low) | ↓ (low) | Normal | ↑↑↑ (markedly elevated) | Severe | | **Argininosuccinate synthetase II** | ↑↑ (elevated) | ↑ (elevated) | Normal | Normal | Mild (cytoplasmic form) | ### Biochemical Mechanism 1. **ASS catalyzes:** Citrulline + Aspartate + ATP → Argininosuccinate + AMP + PPi 2. **Without ASS:** - Citrulline accumulates (cannot be converted to argininosuccinate) → **elevated plasma citrulline** - Arginine cannot be synthesized → **very low plasma arginine** - Ammonia accumulates (cannot be fully processed) → **hyperammonemia** - Orotic acid remains normal (no backup of carbamoyl phosphate) ### Clinical Pearl **Clinical Pearl:** Citrullinemia type I (ASS deficiency) classically presents in infancy with: - Failure to thrive and developmental delay - Recurrent vomiting and lethargy - Consanguineous family history (autosomal recessive) - **Crystalline lens opacities** (cataracts) — unique to this defect - Hepatomegaly may develop **High-Yield:** The **combination of high citrulline + low arginine + normal orotic acid** is pathognomonic for ASS deficiency and distinguishes it from all other urea cycle defects. ### Management 1. Protein restriction 2. Arginine supplementation (essential — arginine is now a conditionally essential amino acid) 3. Nitrogen scavenging agents (sodium benzoate, sodium phenylbutyrate) 4. Regular monitoring of ammonia and amino acid levels **Mnemonic:** **"CIT-LOW ARG"** = **CIT**rulline elevated, **LOW ARG**inine = **ASS** deficiency (Argininosuccinate Synthetase) [cite:Robbins 10e Ch 7]