A 3-day-old male neonate born to non-consanguineous parents presents with poor feeding, lethargy, and vomiting. On examination, he is hypotonic and has a weak cry. Laboratory investigations reveal ammonia level of 380 µmol/L (normal <50), normal blood glucose, normal liver function tests, and normal organic acids on urine chromatography. Plasma amino acid analysis shows elevated glutamine and alanine. A diagnosis of urea cycle disorder is suspected. Which enzyme deficiency is most likely responsible, given the clinical presentation and biochemical pattern?

Explanation

## Clinical Presentation Analysis This neonate presents with the classic features of a urea cycle disorder (UCD): hyperammonemia, neurological symptoms (lethargy, hypotonia, poor feeding), and elevated plasma glutamine and alanine. ### Key Biochemical Clues **High-Yield:** The combination of: - Severe hyperammonemia (380 µmol/L) in a neonate - Normal organic acids (rules out organic acidemias) - Normal glucose (rules out hypoglycemia as primary problem) - Elevated glutamine and alanine (ammonia scavenging amino acids) points to a primary urea cycle defect. ### Why OTC Deficiency? **Key Point:** Ornithine transcarbamylase (OTC) deficiency is the **most common urea cycle disorder**, accounting for ~50% of all UCDs. It presents as: - Neonatal-onset hyperammonemia (typically day 2–4 of life) - Severe neurological symptoms - Male predominance (X-linked inheritance; hemizygous males more severely affected) - Normal carbamoyl phosphate levels (substrate accumulation upstream) **Clinical Pearl:** In OTC deficiency, carbamoyl phosphate cannot be converted to citrulline, so carbamoyl phosphate is shunted to pyrimidine synthesis, leading to orotic acid in urine (though not always tested acutely). The ammonia cannot be incorporated into the urea cycle, causing severe hyperammonemia. ### Urea Cycle Enzyme Defects — Differential | Enzyme | Inheritance | Neonatal Onset | Hyperammonemia Severity | Plasma Citrulline | Urine Orotic Acid | |--------|-------------|----------------|------------------------|-------------------|-------------------| | **CPS I** | AR | Yes | Severe | ↓ | Normal | | **OTC** | XL | Yes | Severe | ↓ | ↑↑ (pathognomonic) | | **Argininosuccinate synthetase** | AR | Yes | Moderate–severe | ↑ | Normal | | **Arginase** | AR | Late onset | Mild–moderate | ↑↑ | Normal | **Mnemonic:** **NAGS** — **N**-acetylglutamate synthase; **A**rgininosuccinate lyase; **G**lucose-6-phosphatase; **S**ynthetase (carbamoyl phosphate synthetase I). These are the less common defects; OTC is the **most common**. ### Why This Case Points to OTC 1. **X-linked inheritance pattern** — male neonate, early-onset severe hyperammonemia 2. **No organic acidemia** — rules out CPS I (which would also present similarly but is autosomal recessive) 3. **Elevated glutamine/alanine** — ammonia scavenging, seen in all UCDs but most common in OTC 4. **Neonatal presentation** — typical for OTC (day 2–4), whereas arginase deficiency often presents later in childhood ## Management Implications **High-Yield:** OTC deficiency management includes: - Protein restriction - Ammonia-lowering agents (sodium benzoate, sodium phenylbutyrate) - Arginine supplementation (bypasses the block, provides substrate for downstream cycle) - Liver transplantation (definitive cure) in severe cases [cite:Robbins 10e Ch 7]