A 5-year-old girl presents with acute encephalopathy, hyperammonemia (240 µmol/L), and normal plasma citrulline levels. Genetic testing reveals argininosuccinate lyase (ASL) deficiency. Regarding the biochemical and clinical features of ASL deficiency, all of the following are true EXCEPT:

Explanation

## ASL Deficiency: Biochemistry and Clinical Correlations ### The Correct Answer (Option B is WRONG) **Key Point:** In ASL (argininosuccinate lyase) deficiency, **citrulline levels are NOT elevated** — this is the false statement. The question stem itself confirms "normal plasma citrulline levels," which is the expected finding in ASL deficiency. ASL acts on argininosuccinate (converting it to arginine + fumarate), not directly on citrulline. Citrulline is converted to argininosuccinate by ASS (argininosuccinate synthetase), which is upstream of ASL. Since ASS is intact, citrulline is consumed normally to form argininosuccinate; it is **argininosuccinate** that accumulates, not citrulline. Option B is doubly incorrect: (1) citrulline is not elevated in ASL deficiency, and (2) the phrasing "enzyme downstream of citrulline is blocked" misidentifies the enzymatic relationship — ASL acts on argininosuccinate, not directly downstream of citrulline in a way that would cause citrulline accumulation. ### Biochemical Block in ASL Deficiency ``` NH₃ → Carbamoyl phosphate → Citrulline → Argininosuccinate →[ASL blocked]→ Arginine + Fumarate ``` - **Citrulline** is converted to argininosuccinate by ASS (intact in ASL deficiency) → citrulline levels are **normal or only mildly elevated** - **Argininosuccinate** accumulates and is excreted in urine (detected on urine amino acid chromatography — no characteristic odor) - **Arginine** synthesis is blocked → plasma arginine is low ### Why the Other Options Are TRUE (and thus not the answer) | Option | Statement | Verdict | |--------|-----------|---------| | **A** | Argininosuccinate accumulates and is excreted in urine | ✅ TRUE — characteristic finding; no fruity odor (fruity odor = MSUD) | | **C** | Plasma arginine is low → recurrent infections, poor wound healing | ✅ TRUE — arginine deficiency impairs T-cell function and wound healing | | **D** | Hyperammonemia milder than OTC deficiency | ✅ TRUE — ammonia is still incorporated into carbamoyl phosphate and citrulline; block is downstream | ### High-Yield Distinction: Urea Cycle Enzyme Deficiencies | Deficiency | Elevated metabolite | Citrulline | |------------|-------------------|------------| | OTC | Orotic acid, NH₃ | Low/absent | | CPS-I | NH₃ | Low/absent | | ASS (Citrullinemia type I) | Citrulline | Very high | | **ASL** | **Argininosuccinate** | **Normal or mildly elevated** | | Arginase | Arginine | Elevated | ### Clinical Pearl **High-Yield:** The diagnostic clue for ASL deficiency is **elevated argininosuccinate in urine/plasma** with **normal or near-normal citrulline** — distinguishing it from citrullinemia (ASS deficiency) where citrulline is markedly elevated. (Reference: Scriver's Metabolic Basis of Inherited Disease; Harper's Illustrated Biochemistry, 31st ed.) **Mnemonic:** ASL deficiency → **A**rgininosuccinate **S**tacks up, **L**ow arginine results.