A 6-month-old female infant of consanguineous parents presents with recurrent episodes of vomiting, irritability, and developmental delay. During a febrile illness, she develops seizures and altered consciousness. Blood ammonia is 520 µmol/L. Plasma amino acid analysis shows: citrulline 45 µmol/L (normal 20–50), arginine 25 µmol/L (normal 40–130), and elevated glutamine. Urine orotic acid is normal. Urine contains high levels of argininosuccinate. What is the most likely diagnosis?

Explanation

## Clinical and Biochemical Analysis **Key Point:** The combination of **high urine argininosuccinate**, **normal citrulline** (45 µmol/L — within normal range 20–50), **low arginine**, and **normal urine orotic acid** is pathognomonic for **Argininosuccinate Lyase (ASL) deficiency**. ### Why This Is Argininosuccinate Lyase Deficiency ``` Carbamoyl phosphate + Ornithine → Citrulline Citrulline + Aspartate → Argininosuccinate [via ASS — INTACT] Argininosuccinate → Arginine + Fumarate [via ASL — BLOCKED] ``` When ASL is deficient: - **Argininosuccinate accumulates** and is excreted in large amounts in urine - **Citrulline remains normal** (the step forming citrulline is intact; citrulline is consumed normally to form argininosuccinate) - **Arginine is low** because the pathway is blocked before arginine synthesis - **Urine orotic acid is normal** (carbamoyl phosphate is still being utilized in the cycle) - **Ammonia is severely elevated** because the urea cycle cannot complete ### Plasma Amino Acid Profile: The Diagnostic Signature | Parameter | ASL Deficiency | ASS Deficiency (Citrullinemia I) | OTC Deficiency | |---|---|---|---| | **Citrulline** | Normal or mildly ↑ | ↑↑↑ (markedly elevated, >1000 µmol/L) | ↓ (low) | | **Arginine** | ↓ (low) | ↓ (low) | ↓ (low) | | **Urine orotic acid** | Normal to mildly ↑ | Normal | ↑↑↑ (markedly elevated) | | **Urine argininosuccinate** | ↑↑↑ (pathognomonic) | Normal | Normal | | **Ammonia** | ↑↑↑ (severe) | ↑↑↑ (severe) | ↑↑↑ (severe) | **High-Yield:** The **citrulline level of 45 µmol/L is within the normal range (20–50 µmol/L)** — this rules out Citrullinemia type I (ASS deficiency), where citrulline is dramatically elevated (often >1000 µmol/L). The **hallmark finding here is high urine argininosuccinate with normal citrulline**. ### Why Not the Other Options? - **OTC Deficiency (A):** Urine orotic acid would be markedly elevated (carbamoyl phosphate shunted to pyrimidine pathway); citrulline would be low; no argininosuccinate in urine. - **CPS-I Deficiency (B):** Urine orotic acid would be normal/low; citrulline would be very low; no argininosuccinate in urine. This is the "bare" urea cycle defect. - **ASS Deficiency / Citrullinemia I (D):** Citrulline would be dramatically elevated (>1000 µmol/L), not 45 µmol/L. Argininosuccinate would not accumulate in urine because the block is upstream of argininosuccinate formation. ### Clinical Features Explained - **Recurrent vomiting and irritability:** Chronic hyperammonemia (520 µmol/L is severe) - **Developmental delay:** Neurotoxic effects of ammonia on the developing brain - **Seizures during febrile illness:** Fever increases protein catabolism → ammonia surge → cerebral edema and seizures - **Consanguineous parents:** ASL deficiency is autosomal recessive (gene: *ASL*, chromosome 7q11) **Clinical Pearl:** ASL deficiency is also associated with **trichorrhexis nodosa** (brittle hair) — a unique extra-urea-cycle feature due to arginine deficiency affecting hair shaft formation. ## Management 1. **Protein restriction** (0.8–1.0 g/kg/day) 2. **Arginine supplementation** (essential amino acid in this defect; replenishes the cycle) 3. **Sodium benzoate or phenylbutyrate** (alternative pathway for ammonia removal) 4. **Liver transplantation** in severe cases [cite: Lehninger Principles of Biochemistry 8e Ch 22; Harrison 21e Ch 352; Scriver's Metabolic Basis of Inherited Disease]