Catabolism of purine nucleotides MCQs With Answer — This concise, keyword-rich introduction is tailored for B. Pharm students studying purine catabolism pathways, enzymes, regulation, clinical correlations, and pharmacological inhibitors. You will review key topics such as degradation of AMP and GMP to hypoxanthine, xanthine and uric acid, the roles of xanthine oxidase, adenosine deaminase, purine nucleoside phosphorylase, and salvage enzymes like HGPRT. Clinical links include gout, hyperuricemia, Lesch-Nyhan syndrome and drugs like allopurinol and febuxostat. Clear focus on mechanisms, lab markers and therapeutic targets prepares you for exams and practice. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. Which enzyme catalyzes the final step in purine catabolism converting xanthine to uric acid?
- Xanthine oxidase
- Adenosine deaminase
- Purine nucleoside phosphorylase
- HGPRT (hypoxanthine-guanine phosphoribosyltransferase)
Correct Answer: Xanthine oxidase
Q2. The immediate product formed from deamination of adenosine in purine catabolism is:
- Guanosine
- Inosine
- Hypoxanthine
- Adenine
Correct Answer: Inosine
Q3. Hypoxanthine is oxidized to xanthine by which enzyme during purine degradation?
- Xanthine oxidase
- Adenosine deaminase
- Purine nucleoside phosphorylase
- Adenine deaminase
Correct Answer: Xanthine oxidase
Q4. Lesch-Nyhan syndrome results from deficiency of which enzyme affecting purine salvage?
- HGPRT
- APR T
- PNP
- Xanthine oxidase
Correct Answer: HGPRT
Q5. Which product accumulates in HGPRT deficiency leading to hyperuricemia?
- Adenosine
- Hypoxanthine and guanine
- Xanthine oxidase
- Urea
Correct Answer: Hypoxanthine and guanine
Q6. Allopurinol lowers uric acid by inhibiting which enzyme?
- Adenosine deaminase
- Xanthine oxidase
- HGPRT
- Purine nucleoside phosphorylase
Correct Answer: Xanthine oxidase
Q7. Which compound is the major end product of purine catabolism in humans?
- Allantoin
- Uric acid
- Ammonia
- Urea
Correct Answer: Uric acid
Q8. Which enzyme converts inosine to hypoxanthine during purine catabolism?
- Purinase
- Purine nucleoside phosphorylase (PNP)
- Adenosine deaminase
- HGPRT
Correct Answer: Purine nucleoside phosphorylase (PNP)
Q9. Which of the following is a substrate for HGPRT in the salvage pathway?
- Adenine
- Guanine and hypoxanthine
- Inosine
- Uric acid
Correct Answer: Guanine and hypoxanthine
Q10. Deficiency of purine nucleoside phosphorylase (PNP) primarily affects which cell type leading to immunodeficiency?
- Neutrophils
- T lymphocytes
- B lymphocytes
- Red blood cells
Correct Answer: T lymphocytes
Q11. Which drug is a xanthine oxidase inhibitor used to treat gout besides allopurinol?
- Febuxostat
- Probenecid
- Colchicine
- Allopurinase
Correct Answer: Febuxostat
Q12. AMP deaminase acts on AMP to form:
- Adenosine
- IMP (inosine monophosphate)
- GMP
- Uric acid
Correct Answer: IMP (inosine monophosphate)
Q13. Which nucleotide is directly dephosphorylated to nucleoside inosine before conversion to hypoxanthine?
- IMP
- AMP
- GMP
- ADP
Correct Answer: IMP
Q14. Which enzyme deficiency causes severe combined immunodeficiency due to toxic accumulation of deoxyadenosine metabolites?
- Adenosine deaminase (ADA) deficiency
- Purinase deficiency
- Xanthine oxidase deficiency
- HGPRT deficiency
Correct Answer: Adenosine deaminase (ADA) deficiency
Q15. Uric acid crystals in joints trigger gout mainly via activation of:
- Platelets
- Complement system only
- NLRP3 inflammasome and IL-1β mediated inflammation
- B cell antibody production
Correct Answer: NLRP3 inflammasome and IL-1β mediated inflammation
Q16. Which of the following increases purine catabolism and uric acid production?
- High intake of vitamin C
- Excess dietary purines and rapid cell turnover
- Allopurinol therapy
- Hypouricemia
Correct Answer: Excess dietary purines and rapid cell turnover
Q17. Conversion of guanine to xanthine requires which enzyme?
- Guanine deaminase (guanase)
- Xanthine oxidase directly on guanine
- Adenosine deaminase
- HGPRT
Correct Answer: Guanine deaminase (guanase)
Q18. In humans, inability to further degrade uric acid to allantoin is due to the absence of which enzyme?
- Uricase (urate oxidase)
- Xanthine oxidase
- Adenosine deaminase
- Purine nucleoside phosphorylase
Correct Answer: Uricase (urate oxidase)
Q19. Which laboratory finding is most characteristic of active gout due to purine catabolism?
- Low serum uric acid
- High serum uric acid (hyperuricemia)
- Elevated urea only
- Low xanthine levels
Correct Answer: High serum uric acid (hyperuricemia)
Q20. Which salvage enzyme recycles adenine to AMP?
- HGPRT
- Adenine phosphoribosyltransferase (APRT)
- Purinase
- Adenosine deaminase
Correct Answer: Adenine phosphoribosyltransferase (APRT)
Q21. Which intermediate links the deamination of AMP and the formation of inosine during purine breakdown?
- IMP
- GMP
- Urea
- Allantoin
Correct Answer: IMP
Q22. The major route of purine disposal in most mammals (not humans) converts uric acid into what product via uricase?
- Urea
- Allantoin
- Ammonia
- Hypoxanthine
Correct Answer: Allantoin
Q23. Which enzyme directly generates hypoxanthine from inosine?
- Nucleoside phosphorylase (PNP)
- Xanthine oxidase
- HGPRT
- Adenine phosphoribosyltransferase
Correct Answer: Nucleoside phosphorylase (PNP)
Q24. Which of the following is a clinical use of allopurinol related to purine catabolism?
- Stimulate uricase
- Reduce uric acid production in gout and tumor lysis syndrome
- Increase purine synthesis
- Activate HGPRT
Correct Answer: Reduce uric acid production in gout and tumor lysis syndrome
Q25. Purine nucleotide phosphorylase deficiency results in accumulation of which type of nucleosides?
- Pyrimidine nucleosides
- Purine nucleosides such as inosine and guanosine
- Allantoic acid
- Uric acid only
Correct Answer: Purine nucleosides such as inosine and guanosine
Q26. Overproduction of uric acid due to increased purine synthesis is often associated with which genetic enzyme defect?
- HGPRT overactivity
- PRPP synthetase hyperactivity
- Xanthine oxidase deficiency
- Adenine deaminase deficiency
Correct Answer: PRPP synthetase hyperactivity
Q27. Which metabolite is elevated in Lesch-Nyhan syndrome and contributes to neurological symptoms?
- Uric acid only
- PRPP and uric acid due to HGPRT deficiency causing purine overproduction
- Allantoin accumulation
- Excess xanthine clearance
Correct Answer: PRPP and uric acid due to HGPRT deficiency causing purine overproduction
Q28. Which cofactor is required by xanthine oxidase for catalysis?
- Biotin
- Molybdenum (molybdopterin) and iron-sulfur, FAD
- Vitamin B12
- Thiamine pyrophosphate
Correct Answer: Molybdenum (molybdopterin) and iron-sulfur, FAD
Q29. In purine catabolism, which pathway step produces hydrogen peroxide as a byproduct contributing to oxidative stress?
- Xanthine oxidase activity converting hypoxanthine to xanthine and xanthine to uric acid
- Adenosine deaminase activity
- HGPRT salvage
- PUR5 step of synthesis
Correct Answer: Xanthine oxidase activity converting hypoxanthine to xanthine and xanthine to uric acid
Q30. Which of the following drugs increases renal excretion of uric acid rather than inhibiting its production?
- Allopurinol
- Probenecid
- Febuxostat
- Rasburicase
Correct Answer: Probenecid
Q31. Which enzyme deficiency causes accumulation of deoxyguanosine and leads to T-cell immunodeficiency?
- Purinase deficiency
- Purin e nucleoside phosphorylase (PNP) deficiency
- Adenosine deaminase deficiency
- Xanthine oxidase deficiency
Correct Answer: Purin e nucleoside phosphorylase (PNP) deficiency
Q32. Which metabolite directly precipitates as monosodium urate crystals in gout?
- Allantoin
- Monosodium urate (uric acid salt)
- Guanine crystals
- Xanthine stones
Correct Answer: Monosodium urate (uric acid salt)
Q33. Which diagnostic test directly measures increased purine breakdown leading to hyperuricemia?
- Serum uric acid concentration
- Serum glucose
- Serum creatinine kinase
- Serum bilirubin
Correct Answer: Serum uric acid concentration
Q34. Which enzyme converts guanosine to guanine in purine catabolism?
- Purinase
- Purine nucleoside phosphorylase (PNP)
- Xanthine oxidase
- HGPRT
Correct Answer: Purine nucleoside phosphorylase (PNP)
Q35. The role of PRPP in purine metabolism is primarily to:
- Degrade purines to uric acid
- Provide ribose-phosphate for purine nucleotide synthesis and salvage reactions
- Act as a xanthine oxidase inhibitor
- Convert uric acid to allantoin
Correct Answer: Provide ribose-phosphate for purine nucleotide synthesis and salvage reactions
Q36. Which clinical condition increases risk of uric acid nephrolithiasis due to low urinary pH?
- Alkaline urine from vegetarian diet
- Acidic urine associated with high purine load and metabolic syndrome
- Hyperventilation causing alkalosis
- Hypouricemia
Correct Answer: Acidic urine associated with high purine load and metabolic syndrome
Q37. Rasburicase is used in tumor lysis syndrome to:
- Inhibit xanthine oxidase
- Recombinantly convert uric acid to allantoin
- Block HGPRT activity
- Increase PRPP levels
Correct Answer: Recombinantly convert uric acid to allantoin
Q38. Which of the following best describes an effect of allopurinol therapy on purine metabolites?
- Increases uric acid and decreases xanthine
- Reduces uric acid and increases xanthine and hypoxanthine levels
- Increases guanine levels only
- Has no effect on purine metabolites
Correct Answer: Reduces uric acid and increases xanthine and hypoxanthine levels
Q39. Which enzyme deficiency leads to accumulation of adenosine and deoxyadenosine causing lymphotoxicity?
- PNP deficiency
- Adenosine deaminase (ADA) deficiency
- Xanthine oxidase deficiency
- HGPRT deficiency
Correct Answer: Adenosine deaminase (ADA) deficiency
Q40. Which statement about human purine catabolism is correct?
- Humans convert uric acid to allantoin efficiently
- Humans lack uricase, so uric acid is the final product
- Purine catabolism occurs only in liver mitochondria
- Uric acid is water-soluble and easily excreted regardless of pH
Correct Answer: Humans lack uricase, so uric acid is the final product
Q41. Which enzyme participates in the conversion of adenine nucleotides to uric acid by deaminating AMP to IMP?
- Adenosine deaminase
- AMP deaminase
- Xanthine oxidase
- Guanine deaminase
Correct Answer: AMP deaminase
Q42. A key regulatory step connecting purine synthesis and salvage involves:
- Allantoin production
- Feedback inhibition by IMP, GMP and AMP on synthetic enzymes and salvage via HGPRT
- Xanthine oxidase activation
- Increased uric acid excretion only
Correct Answer: Feedback inhibition by IMP, GMP and AMP on synthetic enzymes and salvage via HGPRT
Q43. Which enzyme converts xanthosine to xanthine in the catabolic pathway?
- Purine nucleoside phosphorylase (PNP)
- Xanthine oxidase
- HGPRT
- Adenine phosphoribosyltransferase
Correct Answer: Purine nucleoside phosphorylase (PNP)
Q44. Which urinary metabolite increases when xanthine oxidase is inhibited by allopurinol?
- Uric acid
- Xanthine
- Allantoin
- Guanine only
Correct Answer: Xanthine
Q45. Which genetic defect leads to self-mutilation behaviors, gout, and neurological dysfunction in children?
- HGPRT deficiency (Lesch-Nyhan syndrome)
- Adenosine deaminase deficiency
- PNP deficiency
- Xanthine oxidase deficiency
Correct Answer: HGPRT deficiency (Lesch-Nyhan syndrome)
Q46. Which metabolic circumstance favors higher uric acid production from increased nucleotide turnover?
- Stable cell populations
- Hemolysis, tumor lysis syndrome, rapid cell proliferation
- Low protein diet
- High uricase activity in humans
Correct Answer: Hemolysis, tumor lysis syndrome, rapid cell proliferation
Q47. Which molecule is a competitive inhibitor of xanthine oxidase used clinically?
- Colchicine
- Allopurinol (via oxypurinol metabolite)
- Probenecid
- Febuxostat is noncompetitive only
Correct Answer: Allopurinol (via oxypurinol metabolite)
Q48. Which enzyme deficiency is more likely to present with recurrent severe infections and low T-cell counts due to purine metabolism disruption?
- HGPRT deficiency
- Adenosine deaminase (ADA) deficiency
- Xanthine oxidase deficiency
- Guanine deaminase deficiency
Correct Answer: Adenosine deaminase (ADA) deficiency
Q49. Which statement about hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is correct?
- It degrades uric acid to allantoin
- It salvages hypoxanthine and guanine to IMP and GMP respectively
- It converts adenine to AMP
- It produces xanthine from guanine
Correct Answer: It salvages hypoxanthine and guanine to IMP and GMP respectively
Q50. For B. Pharm students, understanding purine catabolism is critical because it directly informs:
- Design of antibiotics only
- Clinical pharmacology of antigout agents, enzyme-targeted therapies, and interpretation of metabolic disorders
- Only nutritional advice
- How to synthesize allopurinol in lab
Correct Answer: Clinical pharmacology of antigout agents, enzyme-targeted therapies, and interpretation of metabolic disorders
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