Introduction: The catabolism of tyrosine is a vital biochemical pathway for B. Pharm students studying amino acid metabolism, enzymology, and metabolic disorders. This SEO-friendly guide focuses on key enzymes (tyrosine aminotransferase, p‑hydroxyphenylpyruvate dioxygenase, homogentisate 1,2‑dioxygenase, maleylacetoacetate isomerase, fumarylacetoacetate hydrolase), cofactors (PLP, iron, glutathione), pathway intermediates (p‑hydroxyphenylpyruvate, homogentisic acid, fumarylacetoacetate) and clinical conditions such as alkaptonuria and tyrosinemia types I–III. Clear understanding of metabolites, diagnostic markers and therapeutic approaches (e.g., nitisinone) is essential for exam success. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which primary products result from complete catabolism of tyrosine?
- Fumarate and acetoacetate
- Oxaloacetate and acetyl‑CoA
- Succinyl‑CoA and pyruvate
- Malate and citrate
Correct Answer: Fumarate and acetoacetate
Q2. The enzyme that catalyzes the transamination of tyrosine to p‑hydroxyphenylpyruvate is:
- Tyrosine aminotransferase (TAT)
- Phenylalanine hydroxylase
- Homogentisate 1,2‑dioxygenase
- Fumarylacetoacetate hydrolase
Correct Answer: Tyrosine aminotransferase (TAT)
Q3. Tyrosine aminotransferase requires which cofactor for activity?
- Pyridoxal phosphate (PLP)
- Biotin
- Thiamine pyrophosphate (TPP)
- Vitamin C
Correct Answer: Pyridoxal phosphate (PLP)
Q4. Accumulation of homogentisic acid due to an enzyme defect leads to which disorder?
- Alkaptonuria
- Tyrosinemia type I
- Phenylketonuria
- Maple syrup urine disease
Correct Answer: Alkaptonuria
Q5. The enzyme deficient in alkaptonuria is:
- Homogentisate 1,2‑dioxygenase (HGD)
- Maleylacetoacetate isomerase
- Tyrosine aminotransferase
- Fumarylacetoacetate hydrolase
Correct Answer: Homogentisate 1,2‑dioxygenase (HGD)
Q6. Which intermediate accumulates in tyrosinemia type I and is diagnostic when detected in urine?
- Succinylacetone
- Homogentisic acid
- p‑Hydroxyphenylpyruvate
- Tyrosine‑N‑acetyl conjugate
Correct Answer: Succinylacetone
Q7. Tyrosinemia type I is caused by deficiency of which enzyme?
- Fumarylacetoacetate hydrolase (FAH)
- Tyrosine aminotransferase
- p‑Hydroxyphenylpyruvate dioxygenase
- Phenylalanine hydroxylase
Correct Answer: Fumarylacetoacetate hydrolase (FAH)
Q8. Which drug is used to treat hereditary tyrosinemia by inhibiting an upstream enzyme?
- Nitisinone (NTBC)
- Allopurinol
- Vitamin B6 supplementation
- Tetrahydrobiopterin (BH4)
Correct Answer: Nitisinone (NTBC)
Q9. Nitisinone inhibits which enzyme in tyrosine catabolism?
- p‑Hydroxyphenylpyruvate dioxygenase (HPPD)
- Tyrosine aminotransferase
- Homogentisate 1,2‑dioxygenase
- Fumarylacetoacetate hydrolase
Correct Answer: p‑Hydroxyphenylpyruvate dioxygenase (HPPD)
Q10. Which cofactor or cosubstrate is directly required by p‑hydroxyphenylpyruvate dioxygenase?
- Molecular oxygen (O2) and Fe2+ (iron)
- Biotin and ATP
- FAD and NADH
- S‑adenosylmethionine (SAM)
Correct Answer: Molecular oxygen (O2) and Fe2+ (iron)
Q11. The conversion of maleylacetoacetate to fumarylacetoacetate requires which type of cofactor?
- Glutathione‑dependent isomerization
- FAD‑dependent oxidation
- ATP‑dependent phosphorylation
- Biotin carboxylation
Correct Answer: Glutathione‑dependent isomerization
Q12. Maleylacetoacetate isomerase deficiency would most directly affect formation of which product?
- Fumarylacetoacetate
- Homogentisic acid
- p‑Hydroxyphenylpyruvate
- Succinylacetone
Correct Answer: Fumarylacetoacetate
Q13. Which metabolite is directly converted to homogentisic acid in tyrosine catabolism?
- p‑Hydroxyphenylpyruvate
- 4‑Hydroxyphenylacetate
- Tyrosine methyl ester
- Phenylpyruvate
Correct Answer: p‑Hydroxyphenylpyruvate
Q14. Alkaptonuria classically presents clinically with which sign due to homogentisic acid deposition?
- Darkening of urine on standing and ochronotic pigmentation of cartilage
- Hypopigmented skin patches
- Sweet odor of urine
- Cherry‑red macula
Correct Answer: Darkening of urine on standing and ochronotic pigmentation of cartilage
Q15. Which inheritance pattern is most common for disorders like tyrosinemia and alkaptonuria?
- Autosomal recessive
- Autosomal dominant
- X‑linked recessive
- Maternal mitochondrial inheritance
Correct Answer: Autosomal recessive
Q16. Fumarylacetoacetate hydrolase catalyzes which type of reaction?
- Hydrolytic cleavage producing fumarate and acetoacetate
- Oxidative decarboxylation to acetyl‑CoA
- Transamination to form glutamate
- Phosphorylation to activate metabolites
Correct Answer: Hydrolytic cleavage producing fumarate and acetoacetate
Q17. Elevated levels of tyrosine in plasma with keratitis and skin lesions suggest deficiency of which enzyme?
- Tyrosine aminotransferase (Type II tyrosinemia)
- Fumarylacetoacetate hydrolase (Type I tyrosinemia)
- Homogentisate dioxygenase (alkaptonuria)
- Phenylalanine hydroxylase (PKU)
Correct Answer: Tyrosine aminotransferase (Type II tyrosinemia)
Q18. In the pathway, which intermediate is directly upstream of maleylacetoacetate?
- Homogentisic acid (after opening of aromatic ring)
- Fumarate
- Tyrosine
- Acetoacetate
Correct Answer: Homogentisic acid (after opening of aromatic ring)
Q19. Which laboratory test is most useful to detect alkaptonuria in a neonate?
- Urine darkening on standing with detection of homogentisic acid
- Serum succinylacetone measurement
- Plasma phenylalanine level
- Urine ketone dipstick only
Correct Answer: Urine darkening on standing with detection of homogentisic acid
Q20. Succinylacetone inhibits which enzyme and can cause secondary complications in tyrosinemia type I?
- δ‑Aminolevulinic acid dehydratase (ALAD), affecting heme synthesis
- Phenylalanine hydroxylase, causing PKU‑like features
- Branched‑chain α‑ketoacid dehydrogenase, causing MSUD‑like features
- Glutathione peroxidase, causing oxidative stress
Correct Answer: δ‑Aminolevulinic acid dehydratase (ALAD), affecting heme synthesis
Q21. A newborn with liver failure, elevated succinylacetone and low tyrosine turnover most likely has a defect in:
- Fumarylacetoacetate hydrolase
- Tyrosine aminotransferase
- Phenylalanine hydroxylase
- Homogentisate 1,2‑dioxygenase
Correct Answer: Fumarylacetoacetate hydrolase
Q22. Which of the following is a biochemical fate of acetoacetate produced from tyrosine catabolism?
- Conversion to acetyl‑CoA and ketone body metabolism
- Direct entry into gluconeogenesis as oxaloacetate
- Storage as glycogen in liver
- Incorporation into nucleotide biosynthesis
Correct Answer: Conversion to acetyl‑CoA and ketone body metabolism
Q23. Which enzyme deficiency causes ochronosis due to pigment deposition in connective tissue?
- Homogentisate 1,2‑dioxygenase deficiency (alkaptonuria)
- Tyrosine aminotransferase deficiency
- Fumarylacetoacetate hydrolase deficiency
- P‑hydroxyphenylpyruvate dioxygenase deficiency
Correct Answer: Homogentisate 1,2‑dioxygenase deficiency (alkaptonuria)
Q24. Which metabolite accumulation is most characteristic of untreated tyrosinemia type I?
- Succinylacetone
- Phenylpyruvate
- Tyrosine glycosides
- Homogentisic acid
Correct Answer: Succinylacetone
Q25. Which analytical method is commonly used to detect specific organic acids like homogentisic acid or succinylacetone in urine?
- Gas chromatography‑mass spectrometry (GC‑MS)
- Western blotting
- ELISA for enzymes
- Hemagglutination assay
Correct Answer: Gas chromatography‑mass spectrometry (GC‑MS)
Q26. Tyrosinemia type II primarily affects which organ systems clinically?
- Eye (keratitis) and skin (hyperkeratotic lesions)
- Renal tubular system only
- Pancreas and endocrine glands
- Respiratory epithelium
Correct Answer: Eye (keratitis) and skin (hyperkeratotic lesions)
Q27. Which enzyme performs the ring‑cleavage of homogentisic acid?
- Homogentisate 1,2‑dioxygenase
- Tyrosine aminotransferase
- p‑Hydroxyphenylpyruvate dioxygenase
- Fumarylacetoacetate hydrolase
Correct Answer: Homogentisate 1,2‑dioxygenase
Q28. Which dietary measure is commonly recommended for managing tyrosinemia disorders?
- Restriction of dietary tyrosine and phenylalanine
- High‑protein diet to enhance metabolism
- High fat ketogenic diet
- Unrestricted diet with vitamin C supplementation
Correct Answer: Restriction of dietary tyrosine and phenylalanine
Q29. Which intermediate indicates impairment at the step catalyzed by p‑hydroxyphenylpyruvate dioxygenase when elevated?
- p‑Hydroxyphenylpyruvate
- Homogentisic acid
- Succinylacetone
- Acetoacetate
Correct Answer: p‑Hydroxyphenylpyruvate
Q30. The final cleavage producing fumarate in tyrosine catabolism funnels carbon into which metabolic cycle?
- Tricarboxylic acid (TCA) cycle
- Glyoxylate cycle
- Urea cycle
- Pentose phosphate pathway
Correct Answer: Tricarboxylic acid (TCA) cycle
Q31. Which statement about tyrosinemia type III is correct?
- It is rare and associated with elevated p‑hydroxyphenylpyruvate and neurological symptoms
- It primarily causes ochronosis like alkaptonuria
- It results from fumarylacetoacetate hydrolase deficiency
- It is treated by high‑tyrosine diet
Correct Answer: It is rare and associated with elevated p‑hydroxyphenylpyruvate and neurological symptoms
Q32. Which vitamin deficiency would impair tyrosine aminotransferase activity indirectly?
- Vitamin B6 (pyridoxine)
- Vitamin C (ascorbate)
- Vitamin D
- Vitamin B12
Correct Answer: Vitamin B6 (pyridoxine)
Q33. Homogentisic acid is derived from which type of chemical transformation of the aromatic ring?
- Oxidative cleavage and ring modification leading to aliphatic intermediates
- Methylation of the ring
- Conjugation with sulfate
- Direct deamination without ring modification
Correct Answer: Oxidative cleavage and ring modification leading to aliphatic intermediates
Q34. Which of the following best describes biochemical therapy with nitisinone?
- Blocks p‑hydroxyphenylpyruvate dioxygenase to prevent downstream toxic metabolites
- Replaces deficient fumarylacetoacetate hydrolase activity
- Enhances homogentisate dioxygenase function
- Supplies cofactors to increase tyrosine aminotransferase activity
Correct Answer: Blocks p‑hydroxyphenylpyruvate dioxygenase to prevent downstream toxic metabolites
Q35. Which metabolite accumulation is responsible for renal tubular dysfunction in tyrosinemia type I?
- Succinylacetone
- Homogentisic acid
- Phenylalanine
- p‑Hydroxyphenylpyruvate
Correct Answer: Succinylacetone
Q36. In laboratory diagnosis, measurement of which plasma amino acid is helpful when screening for disorders of tyrosine catabolism?
- Tyrosine
- Glycine
- Methionine
- Leucine
Correct Answer: Tyrosine
Q37. Which enzyme in tyrosine catabolism is subject to feedback by its substrates or products in clinical therapy?
- p‑Hydroxyphenylpyruvate dioxygenase is pharmacologically inhibited by nitisinone to reduce toxic products
- Tyrosine aminotransferase is activated by nitisinone
- Homogentisate dioxygenase is replaced by dietary cofactors
- Fumarylacetoacetate hydrolase is inhibited by vitamin B6
Correct Answer: p‑Hydroxyphenylpyruvate dioxygenase is pharmacologically inhibited by nitisinone to reduce toxic products
Q38. Which clinical feature is most characteristic of long‑standing alkaptonuria?
- Progressive ochronotic arthropathy with cartilage degeneration
- Acute metabolic crises with hypoglycemia
- Pancreatitis in infancy
- Cardiomyopathy in adolescence
Correct Answer: Progressive ochronotic arthropathy with cartilage degeneration
Q39. Maleylacetoacetate isomerase uses which small molecule in catalysis?
- Reduced glutathione (GSH)
- NADH
- Coenzyme A
- ATP
Correct Answer: Reduced glutathione (GSH)
Q40. Which enzyme deficiency would most likely present with neonatal liver failure and elevated alpha‑fetoprotein?
- Fumarylacetoacetate hydrolase deficiency (tyrosinemia type I)
- Tyrosine aminotransferase deficiency
- Homogentisate dioxygenase deficiency
- p‑Hydroxyphenylpyruvate dioxygenase deficiency
Correct Answer: Fumarylacetoacetate hydrolase deficiency (tyrosinemia type I)
Q41. Which molecule formed from tyrosine catabolism directly enters the TCA cycle?
- Fumarate
- Acetoacetate
- Phenylacetate
- p‑Hydroxybenzoate
Correct Answer: Fumarate
Q42. A metabolic block at homogentisate 1,2‑dioxygenase would predict accumulation of which compound?
- Homogentisic acid (HGA)
- Fumarate
- Acetoacetate
- Succinylacetone
Correct Answer: Homogentisic acid (HGA)
Q43. Which clinical test helps monitor the effectiveness of nitisinone therapy in tyrosinemia?
- Decrease in urinary succinylacetone and clinical improvement
- Increase in homogentisic acid in urine
- Plasma glucose rise
- Serum cholesterol measurement
Correct Answer: Decrease in urinary succinylacetone and clinical improvement
Q44. Which statement about fumarate produced from tyrosine is correct?
- It can be metabolized in the TCA cycle to generate energy
- It is excreted unmetabolized in urine as a toxin
- It is converted directly to glucose via gluconeogenesis
- It is used exclusively for fatty acid synthesis
Correct Answer: It can be metabolized in the TCA cycle to generate energy
Q45. Which biochemical marker is most specific for diagnosing tyrosinemia type I in newborn screening?
- Succinylacetone in blood or urine
- Elevated plasma phenylalanine
- Elevated urine homogentisic acid
- Low plasma tyrosine
Correct Answer: Succinylacetone in blood or urine
Q46. Which organ is primarily affected and often severely damaged in tyrosinemia type I?
- Liver (hepatocellular injury and failure)
- Brain (selective demyelination)
- Pancreas (beta cell destruction)
- Skin (epidermal atrophy)
Correct Answer: Liver (hepatocellular injury and failure)
Q47. Which pathway intermediate can be diverted into ketone body formation?
- Acetoacetate
- Fumarate
- Succinylacetone
- p‑Hydroxyphenylpyruvate
Correct Answer: Acetoacetate
Q48. Which enzyme in tyrosine catabolism is a PLP‑dependent aminotransferase?
- Tyrosine aminotransferase (TAT)
- p‑Hydroxyphenylpyruvate dioxygenase
- Homogentisate 1,2‑dioxygenase
- Fumarylacetoacetate hydrolase
Correct Answer: Tyrosine aminotransferase (TAT)
Q49. A pharmacist counseling parents about nitisinone should explain that it may cause which laboratory change?
- Marked increase in plasma tyrosine due to upstream blockade
- Decrease in plasma tyrosine to undetectable levels
- Elevation of plasma phenylalanine only
- Decrease in urinary homogentisic acid concentration
Correct Answer: Marked increase in plasma tyrosine due to upstream blockade
Q50. Which research or therapeutic strategy is relevant for managing downstream toxicity in tyrosine catabolism disorders?
- Enzyme inhibition upstream (e.g., nitisinone), dietary restriction, and liver transplantation for severe FAH deficiency
- High tyrosine supplementation and avoidance of nitisinone
- Gene silencing of tyrosine aminotransferase exclusively
- Administration of homogentisic acid to compete with toxic metabolites
Correct Answer: Enzyme inhibition upstream (e.g., nitisinone), dietary restriction, and liver transplantation for severe FAH deficiency
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
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