Regarding the biochemical defects and clinical features of amino acidurias, all of the following statements are correct EXCEPT:

Explanation

## Analysis of Amino Acidurias — Biochemical Defects ### Correct Statements (Options 0, 1, 2) **Option 0 — PKU:** - Enzyme defect: Phenylalanine hydroxylase (converts phenylalanine → tyrosine) - Accumulation: Phenylalanine, phenylpyruvate, phenyllactate, phenylacetate - Clinical: Intellectual disability, light skin, musty odour, eczema if untreated **Option 1 — Alkaptonuria:** - Enzyme defect: Homogentisate 1,2-dioxygenase - Accumulation: Homogentisic acid → dark urine on standing (oxidation and polymerization) - Late feature: Ochronosis (dark pigmentation of connective tissue) in 3rd–4th decade - Relatively benign; intellectual disability does NOT occur **Option 2 — Maple Syrup Urine Disease (MSUD):** - Enzyme defect: Branched-chain α-ketoacid dehydrogenase complex - Accumulation: Leucine, isoleucine, valine and their keto-acids - Urine: Sweet/maple syrup odour (due to keto-acids) - Presentation: Severe neurological deterioration if untreated (weeks of life) ### Incorrect Statement (Option 3 — Tyrosinemia Type 1) **Key Point:** Tyrosinemia type 1 (hepatorenal tyrosinemia) caused by deficiency of fumarylacetoacetate hydrolase (FAH) is one of the **most severe** amino acidurias. - **Hepatic manifestations:** Acute liver failure in infancy, cirrhosis, hepatocellular carcinoma - **Renal manifestations:** Fanconi syndrome (hypophosphatemic rickets, renal tubular dysfunction) — **TRUE** - **Neurological manifestations:** Acute neurological crises with seizures, encephalopathy — **DOES cause intellectual disability if untreated** - **Metabolic:** Accumulation of tyrosine, methionine, and toxic intermediates (succinylacetone) The statement claims tyrosinemia type 1 does NOT cause intellectual disability if untreated early — this is **FALSE**. Untreated tyrosinemia type 1 causes severe neurological complications including intellectual disability, seizures, and acute encephalopathic crises. ### Comparative Table | Amino Aciduria | Enzyme Defect | Primary Accumulation | Key Clinical Feature | Intellectual Disability? | | --- | --- | --- | --- | --- | | **PKU** | Phenylalanine hydroxylase | Phenylalanine | Musty odour, light skin, seizures | Yes (if untreated) | | **Alkaptonuria** | Homogentisate 1,2-dioxygenase | Homogentisic acid | Dark urine, ochronosis (late) | No | | **MSUD** | Branched-chain α-ketoacid DH | Leucine, isoleucine, valine | Sweet urine, severe neurodegeneration | Yes (severe, acute) | | **Tyrosinemia I** | Fumarylacetoacetate hydrolase | Tyrosine, succinylacetone | Liver failure, Fanconi, neurological crises | Yes (if untreated) | **High-Yield:** Tyrosinemia type 1 is the **most severe** amino aciduria with multi-organ involvement (liver, kidney, nervous system). Treatment with nitisinone (NTBC) inhibits the pathway upstream and prevents disease progression.