Which is the most common urea cycle enzyme deficiency causing hyperammonemia in neonates and infants?

Explanation

## Epidemiology of Urea Cycle Defects **Key Point:** Ornithine transcarbamylase (OTC) deficiency is the most common urea cycle disorder, accounting for approximately 40–50% of all urea cycle enzyme deficiencies. ### Why OTC Deficiency is Most Common 1. **X-linked inheritance pattern** — The OTC gene is located on the X chromosome, making it the only X-linked urea cycle defect. This increases the frequency of mutations in the population. 2. **High mutation rate** — The OTC locus has a relatively high spontaneous mutation rate. 3. **Variable expressivity** — Heterozygous females can present with symptoms due to skewed X-inactivation, increasing clinical recognition. ### Clinical Presentation - **Neonatal onset:** Typically within 24–72 hours of life with poor feeding, lethargy, vomiting, and seizures. - **Ammonia levels:** Often exceed 1000 µmol/L (normal: 15–45 µmol/L). - **Neurological sequelae:** Hyperammonemia causes cerebral edema, seizures, and permanent neurological damage if untreated. ### Biochemical Basis OTC catalyzes the second step of the urea cycle: $$\text{Carbamoyl phosphate} + \text{Ornithine} \xrightarrow{OTC} \text{Citrulline} + \text{Pi}$$ Deficiency leads to: - Accumulation of carbamoyl phosphate → increased pyrimidine synthesis → orotic aciduria (hallmark finding). - Decreased citrulline production. - Severe hyperammonemia. **High-Yield:** Orotic acid in urine is a **diagnostic clue** for OTC deficiency — it is virtually absent in other urea cycle disorders. ### Comparison with Other Urea Cycle Defects | Enzyme | Frequency | Inheritance | Key Finding | Ammonia | |--------|-----------|-------------|-------------|----------| | OTC | 40–50% | X-linked | Orotic aciduria | ↑↑↑ | | CPS I | 10–15% | Autosomal recessive | Low citrulline | ↑↑↑ | | Argininosuccinate synthetase | 10–15% | Autosomal recessive | Low argininosuccinate | ↑↑ | | Argininosuccinate lyase | 5–10% | Autosomal recessive | ↑ Argininosuccinate | ↑↑ | **Clinical Pearl:** OTC deficiency in heterozygous females may present later in life, triggered by infection, high protein intake, or pregnancy — a common trap in board exams.