In carbamoyl phosphate synthetase I (CPS I) deficiency, which of the following best explains the pattern of hyperammonemia with normal or low orotic acid levels?

Explanation

## CPS I Deficiency: A Proximal Urea Cycle Block ### Mechanism of Hyperammonemia **Key Point:** CPS I catalyzes the **first committed step** of the urea cycle: ammonia + CO₂ + 2 ATP → carbamoyl phosphate. When CPS I is deficient, this step cannot proceed, and ammonia has no entry point into the urea cycle. ### Why Orotic Acid is NOT Elevated **High-Yield:** Orotic acid elevation requires **accumulation of carbamoyl phosphate** (which is shunted to pyrimidine synthesis). In CPS I deficiency, carbamoyl phosphate is **not produced** in the first place, so there is no substrate for pyrimidine synthesis. ### Biochemical Pattern ```mermaid flowchart TD A["Ammonia (NH₃)"]:::outcome --> B{"CPS I present?"}:::decision B -->|"Yes (normal)"| C["Carbamoyl phosphate formed"]:::action C --> D["Enters urea cycle"]:::action B -->|"No (CPS I deficiency)"| E["Carbamoyl phosphate NOT formed"]:::urgent E --> F["Ammonia accumulates"]:::urgent E --> G["Orotic acid NORMAL/LOW"]:::outcome D --> H["Urea formed, ammonia cleared"]:::outcome ``` ### Comparison: CPS I vs OTC Deficiency | Feature | CPS I Deficiency | OTC Deficiency | |---|---|---| | **Block location** | Proximal (step 1) | Proximal (step 2) | | **Ammonia level** | ↑↑↑ Severe | ↑↑↑ Severe | | **Carbamoyl phosphate** | ↓ Low/absent | ↑↑ Accumulates | | **Orotic acid** | Normal/↓ | ↑↑↑ In urine | | **Diagnostic clue** | Hyperammonemia alone | Hyperammonemia + orotic aciduria | **Mnemonic:** **CPS I = Can't Produce Substrate** — no carbamoyl phosphate means no entry of ammonia into the cycle, and no substrate for pyrimidine overflow. **Clinical Pearl:** CPS I deficiency is rare (~5% of urea cycle disorders) and presents with severe neonatal hyperammonemia, but the **absence of orotic aciduria** is the distinguishing laboratory feature. [cite:Lehninger Principles of Biochemistry 7e Ch 18]