Correct Answer: C. Type 1 Tyrosinemia
Fumarylacetoacetate hydrolase (FAH) is the final enzyme in the tyrosine catabolism pathway. Deficiency of this enzyme causes Type 1 Tyrosinemia (hepatorenal tyrosinemia), the most severe form. When FAH is absent, the upstream metabolite fumarylacetoacetate accumulates and is converted to succinylacetone, a potent hepatotoxin that causes acute liver necrosis, cirrhosis, and hepatocellular carcinoma in infancy or early childhood. The disease also presents with renal tubular dysfunction (Fanconi syndrome), neurological crises, and elevated plasma tyrosine levels. This is distinct from other tyrosinemias: Type 2 (oculocutaneous tyrosinemia) results from tyrosine aminotransferase deficiency; Type 3 (mild tyrosinemia) results from 4-hydroxyphenylpyruvate dioxygenase deficiency; Type 4 is extremely rare and involves hepatic tyrosyl-tRNA synthetase deficiency. FAH deficiency is the only form with such severe hepatorenal manifestations and succinylacetone accumulation, making it the pathognomonic marker for Type 1 Tyrosinemia. Indian pediatric centers recognize this as a metabolic emergency requiring nitisinone therapy and dietary tyrosine/phenylalanine restriction.
Why the other options are wrong
A. Type 2 Tyrosinemia — Type 2 Tyrosinemia results from deficiency of tyrosine aminotransferase (TAT), not FAH. It presents with oculocutaneous manifestations (painful keratitis, palmoplantar hyperkeratosis, intellectual disability) but lacks the severe hepatorenal involvement and succinylacetone accumulation seen in Type 1. This is the NBE trap—confusing the enzyme defect with the clinical phenotype. B. Type 4 Tyrosinemia — Type 4 Tyrosinemia is an extremely rare condition caused by deficiency of hepatic tyrosyl-tRNA synthetase, not FAH. It presents with mild hypertyrosinemia and pseudotumor cerebri but is clinically benign compared to Type 1. This option is included to test whether students know the complete classification of tyrosinemias, but FAH deficiency is never associated with Type 4. D. Type 3 Tyrosinemia — Type 3 Tyrosinemia results from deficiency of 4-hydroxyphenylpyruvate dioxygenase (HPPD), an earlier enzyme in the tyrosine degradation pathway, not FAH. It is the mildest form, presenting with hypertyrosinemia and neurological symptoms but without hepatorenal involvement or succinylacetone accumulation. FAH deficiency is never the cause of Type 3.
High-Yield Facts
- FAH deficiency → Type 1 Tyrosinemia (hepatorenal tyrosinemia); most severe form with acute liver necrosis and HCC risk in infancy.
- Succinylacetone accumulation is pathognomonic for Type 1 Tyrosinemia; acts as a hepatotoxin and inhibits δ-ALA dehydratase (porphyria-like crisis).
- Nitisinone (NTBC) is the disease-modifying therapy for Type 1 Tyrosinemia; inhibits HPPD upstream, preventing toxic metabolite formation.
- Type 2 Tyrosinemia = TAT deficiency (oculocutaneous); Type 3 = HPPD deficiency (mild); Type 4 = tyrosyl-tRNA synthetase deficiency (rare, benign).
- Fanconi syndrome (renal tubular dysfunction) is a cardinal feature of Type 1 Tyrosinemia, distinguishing it from other tyrosinemias.
Mnemonics
FAH = Type 1 (Final enzyme = First/Fiercest disease) FAH is the final enzyme in tyrosine catabolism → Type 1 (most severe). Remember: the last enzyme in the pathway, when defective, causes the worst disease. Use this when you see 'fumarylacetoacetate hydrolase' in the stem. Tyrosinemia Types by Enzyme (TAT-HPPD-FAH order) Type 2 = TAT (oculocutaneous); Type 3 = HPPD (mild); Type 1 = FAH (severe hepatorenal). The pathway order is TAT → HPPD → FAH; earlier defects = milder disease.
NBE Trap
NBE pairs "fumarylacetoacetate hydrolase" with tyrosinemia types to test whether students memorize the specific enzyme-to-type mapping. Students who only know "tyrosinemia = liver disease" may guess Type 1 correctly but for the wrong reason; those who confuse enzyme names across types will select Type 2 or 3.
Clinical Pearl
In Indian pediatric practice, Type 1 Tyrosinemia presents as a neonatal metabolic emergency with acute liver failure, renal tubular dysfunction, and neurological crises (porphyria-like attacks from succinylacetone). Early diagnosis via succinylacetone in urine and nitisinone initiation can prevent HCC and improve long-term survival—a critical point for NEET PG pediatrics and biochemistry.
_Reference: Robbins Ch. 5 (Genetic Disorders); Harper Biochemistry Ch. 31 (Amino Acid Metabolism); KD Tripathi Pharmacology (Nitisinone therapy)_