A 3-day-old male neonate born to non-consanguineous parents presents with poor feeding, lethargy, and vomiting. On examination, he is hypotonic with weak cry. Laboratory investigations reveal: blood ammonia 280 µmol/L (normal <50), blood pH 7.32, HCO₃⁻ 18 mEq/L, and normal liver function tests. Plasma amino acid profile shows elevated glutamine and alanine with low citrulline. Urine orotic acid is markedly elevated. What is the most likely diagnosis?

Explanation

## Clinical Presentation and Diagnosis **Key Point:** OTC deficiency is the most common urea cycle disorder (X-linked inheritance, ~50% of all urea cycle defects), presenting with hyperammonemia and elevated urinary orotic acid—a pathognomonic finding. ### Why Elevated Urinary Orotic Acid Points to OTC Deficiency In OTC deficiency, carbamoyl phosphate accumulates because it cannot be converted to citrulline. This excess carbamoyl phosphate is shunted into the pyrimidine synthesis pathway, producing excessive orotic acid that is excreted in urine. This is the **diagnostic hallmark**. ### Biochemical Mechanism ```mermaid flowchart TD A[Carbamoyl phosphate + Ornithine]:::outcome --> B{OTC enzyme present?}:::decision B -->|Yes| C[Citrulline formed]:::action B -->|No| D[Carbamoyl phosphate accumulates]:::urgent D --> E[Shunted to pyrimidine synthesis]:::action E --> F[Excess orotic acid produced]:::outcome F --> G[Markedly elevated urinary orotic acid]:::outcome C --> H[Normal urea cycle]:::action ``` ### Laboratory Findings in OTC Deficiency | Finding | OTC Deficiency | CPS I Deficiency | Argininosuccinate Lyase | Argininosuccinate Synthetase | |---------|---|---|---|---| | **Ammonia** | ↑↑ (severe) | ↑↑ (severe) | ↑ (mild-moderate) | ↑↑ (moderate-severe) | | **Urinary orotic acid** | ↑↑↑ (markedly elevated) | Normal/low | Normal | Normal | | **Citrulline** | Low | Low | High | Very high | | **Argininosuccinate** | Normal | Normal | Very high | Normal | | **Inheritance** | X-linked | Autosomal recessive | Autosomal recessive | Autosomal recessive | **High-Yield:** The combination of **hyperammonemia + elevated urinary orotic acid + low citrulline** is pathognomonic for OTC deficiency. ### Clinical Pearl OTC deficiency typically presents in **male neonates** (hemizygous) within 24–72 hours of life with poor feeding, vomiting, lethargy, and hypotonia. Heterozygous females may present later with milder symptoms triggered by infection or high protein intake. ### Plasma Amino Acid Profile in OTC - **Elevated:** Glutamine, alanine (ammonia scavenging pathways activated) - **Low:** Citrulline, arginine (urea cycle blocked at OTC step) - **Normal or low:** Argininosuccinate (not accumulated because it is never formed) **Warning:** Do not confuse OTC deficiency with argininosuccinate lyase deficiency—the latter presents with **high argininosuccinate** and **normal or low orotic acid**, and typically has milder hyperammonemia. ### Management Principles 1. **Acute:** Protein restriction, IV arginine, sodium benzoate (nitrogen scavenger), hemodialysis if ammonia >500 µmol/L 2. **Chronic:** Low-protein diet, arginine supplementation, sodium benzoate or sodium phenylbutyrate 3. **Genetic counseling:** X-linked inheritance; affected males have 100% affected daughters (carriers) and 0% affected sons [cite:Robbins 10e Ch 7]