Nitrogen metabolism MCQs With Answer

Nitrogen metabolism MCQs With Answer offer B.Pharm students a focused, exam-oriented review of amino acid catabolism, transamination, deamination, urea cycle, nitrogen balance, and key enzymes. This concise, keyword-rich introduction links biochemical pathways to pharmacology and clinical biochemistry, emphasizing enzyme cofactors (like PLP and NAD+), regulatory molecules (N-acetylglutamate), inherited defects (e.g., OTC deficiency), ammonia toxicity, and therapeutic measures. Detailed practice questions reinforce hepatic nitrogen disposal, glutamine shuttle, diagnostic markers (BUN, ammonia), and drugs that influence nitrogen handling. Ideal for semester exams and clinical pharmacy contexts, the set deepens conceptual understanding and application. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which cofactor is essential for aminotransferase (transaminase) reactions?

• Pyridoxal phosphate (PLP)
• Nicotinamide adenine dinucleotide (NAD+)
• Biotin
• Tetrahydrofolate (THF)

Correct Answer: Pyridoxal phosphate (PLP)

Q2. Alanine aminotransferase (ALT) catalyzes the transfer of an amino group from alanine to which acceptor?

• Oxaloacetate
• Pyruvate
• Alpha-ketoglutarate
• Glutamate

Correct Answer: Alpha-ketoglutarate

Q3. Which enzyme directly produces free ammonia in amino acid catabolism?

• Glutamine synthetase
• Glutamate dehydrogenase
• Alanine aminotransferase
• Arginase

Correct Answer: Glutamate dehydrogenase

Q4. Where does the first step of the urea cycle (carbamoyl phosphate formation) occur?

• Cytosol
• Mitochondrial matrix
• Endoplasmic reticulum
• Golgi apparatus

Correct Answer: Mitochondrial matrix

Q5. Which molecule provides the second nitrogen atom incorporated into urea?

• Ammonia
• Aspartate
• Glutamine
• Ornithine

Correct Answer: Aspartate

Q6. N-acetylglutamate (NAG) is required for activation of which urea cycle enzyme?

• Ornithine transcarbamylase (OTC)
• Arginase
• Carbamoyl phosphate synthetase I (CPS I)
• Argininosuccinate synthetase (ASS)

Correct Answer: Carbamoyl phosphate synthetase I (CPS I)

Q7. Ornithine transcarbamylase (OTC) deficiency typically leads to accumulation of which metabolite?

• Urea
• Ornithine
• Orotic acid
• Citrulline

Correct Answer: Orotic acid

Q8. Which clinical sign is most directly associated with hyperammonemia?

• Jaundice
• Hepatic encephalopathy
• Hypertension
• Polyuria

Correct Answer: Hepatic encephalopathy

Q9. Glutamine synthetase catalyzes which reaction?

• Glutamate + NH4+ → Glutamine
• Glutamine → Glutamate + NH4+
• Glutamate → α-Ketoglutarate + NH4+
• Alanine → Pyruvate

Correct Answer: Glutamate + NH4+ → Glutamine

Q10. Which enzyme deficiency is X-linked and often presents in neonates with hyperammonemia and high orotic acid?

• Arginase deficiency
• Ornithine transcarbamylase (OTC) deficiency
• Carbamoyl phosphate synthetase II deficiency
• Glutamine synthetase deficiency

Correct Answer: Ornithine transcarbamylase (OTC) deficiency

Q11. Which hepatic enzyme converts arginine to urea and ornithine?

• Arginase
• Argininosuccinate synthase
• Argininosuccinate lyase
• Ornithine decarboxylase

Correct Answer: Arginase

Q12. In peripheral tissues, excess ammonia is usually transported to the liver as which compound?

• Urea
• Glutamine
• Alanine
• Nitrite

Correct Answer: Glutamine

Q13. Which aminotransferase is most liver-specific and commonly measured as a marker of hepatocellular injury?

• Aspartate aminotransferase (AST)
• Alanine aminotransferase (ALT)
• Glutamate dehydrogenase
• Ornithine transcarbamylase

Correct Answer: Alanine aminotransferase (ALT)

Q14. Which drug is associated with causing hyperammonemia by impairing urea cycle function?

• Valproic acid
• Amoxicillin
• Propranolol
• Metformin

Correct Answer: Valproic acid

Q15. The amino group from most amino acids is initially transferred to which keto acid during transamination?

• Pyruvate
• Oxaloacetate
• Alpha-ketoglutarate
• Succinyl-CoA

Correct Answer: Alpha-ketoglutarate

Q16. What is the main nitrogenous waste product excreted by humans?

• Urea
• Uric acid
• Ammonia
• Creatine

Correct Answer: Urea

Q17. Which enzyme uses glutamine as the nitrogen donor in pyrimidine biosynthesis and is cytosolic?

• Carbamoyl phosphate synthetase I (CPS I)
• Carbamoyl phosphate synthetase II (CPS II)
• Ornithine transcarbamylase
• Aspartate transcarbamylase

Correct Answer: Carbamoyl phosphate synthetase II (CPS II)

Q18. Which metabolic fate classifies an amino acid as purely ketogenic?

• Converted to glucose precursors
• Converted to acetyl-CoA or acetoacetate only
• Converted to oxaloacetate
• Converted to α-ketoglutarate

Correct Answer: Converted to acetyl-CoA or acetoacetate only

Q19. Which two amino acids are exclusively ketogenic?

• Leucine and lysine
• Alanine and glycine
• Phenylalanine and tyrosine
• Isoleucine and valine

Correct Answer: Leucine and lysine

Q20. Which assay is commonly used to estimate renal clearance related to muscle-derived nitrogenous waste?

• Serum ammonia
• Blood urea nitrogen (BUN)
• Plasma creatinine
• Urine uric acid

Correct Answer: Plasma creatinine

Q21. Phenylketonuria (PKU) results from deficiency of which enzyme?

• Phenylalanine hydroxylase
• Tyrosine aminotransferase
• Homogentisate oxidase
• Branched-chain α-ketoacid dehydrogenase

Correct Answer: Phenylalanine hydroxylase

Q22. Which transporter/shuttle moves nitrogen from muscle to liver primarily in the form of alanine?

• Cori cycle
• Glutamine shuttle
• Alanine cycle (glucose-alanine cycle)
• Urea shuttle

Correct Answer: Alanine cycle (glucose-alanine cycle)

Q23. Amino acid transamination reactions are generally reversible because:

• They do not require a cofactor
• They form very unstable products
• They involve PLP and near-equilibrium reactions
• They consume ATP irreversibly

Correct Answer: They involve PLP and near-equilibrium reactions

Q24. Which intermediate links the urea cycle and the TCA cycle?

• Fumarate
• Succinate
• Citrate
• Malate

Correct Answer: Fumarate

Q25. Which compound is the immediate precursor of arginine in the urea cycle?

• Citrulline
• Argininosuccinate
• Ornithine
• Carbamoyl phosphate

Correct Answer: Argininosuccinate

Q26. Which enzyme catalyzes the synthesis of argininosuccinate from citrulline and aspartate?

• Argininosuccinate lyase (ASL)
• Argininosuccinate synthase (ASS)
• Arginase
• Ornithine transcarbamylase

Correct Answer: Argininosuccinate synthase (ASS)

Q27. Which laboratory marker rises in blood when the urea cycle is impaired, indicating defective nitrogen disposal?

• Blood glucose
• Serum ammonia
• Total bilirubin
• Plasma lactate

Correct Answer: Serum ammonia

Q28. Which amino acid is a primary amino group donor for synthesis of other amino acids via transamination?

• Glutamate
• Glycine
• Tryptophan
• Cysteine

Correct Answer: Glutamate

Q29. Which therapeutic agent provides an alternative pathway for nitrogen excretion by conjugating with glycine?

• Sodium benzoate
• Lactulose
• Rifaximin
• Sodium bicarbonate

Correct Answer: Sodium benzoate

Q30. Lactulose lowers blood ammonia primarily by which mechanism?

• Inhibiting hepatic glutamine synthetase
• Acidifying colonic contents to trap ammonia as ammonium
• Increasing renal urea excretion
• Blocking intestinal absorption of amino acids

Correct Answer: Acidifying colonic contents to trap ammonia as ammonium

Q31. Which vitamin deficiency (commonly due to isoniazid) impairs transamination reactions?

• Vitamin B12
• Vitamin B6 (pyridoxine)
• Folate
• Biotin

Correct Answer: Vitamin B6 (pyridoxine)

Q32. Which of the following is NOT a urea cycle intermediate?

• Citrulline
• Argininosuccinate
• Homocysteine
• Ornithine

Correct Answer: Homocysteine

Q33. Increased blood urea nitrogen (BUN) with normal creatinine suggests which likely cause?

• Prerenal azotemia (e.g., dehydration)
• Intrinsic renal failure
• Acute tubular necrosis
• Muscle injury

Correct Answer: Prerenal azotemia (e.g., dehydration)

Q34. Which enzyme deficiency causes elevated phenylalanine and decreased tyrosine levels?

• Tyrosine aminotransferase deficiency
• Phenylalanine hydroxylase deficiency
• Homogentisate oxidase deficiency
• Branched-chain aminotransferase deficiency

Correct Answer: Phenylalanine hydroxylase deficiency

Q35. In the brain, excess ammonia is detoxified primarily by converting glutamate to:

• Asparagine
• Glutamine
• Alanine
• Urea

Correct Answer: Glutamine

Q36. Which enzyme links amino acid nitrogen metabolism to nucleotide biosynthesis via carbamoyl phosphate?

• Carbamoyl phosphate synthetase I (CPS I)
• Carbamoyl phosphate synthetase II (CPS II)
• Ornithine transcarbamylase
• Arginase

Correct Answer: Carbamoyl phosphate synthetase II (CPS II)

Q37. Which amino acid is the primary nitrogen donor for purine and pyrimidine biosynthesis?

• Glutamine
• Lysine
• Tyrosine
• Phenylalanine

Correct Answer: Glutamine

Q38. Which statement about the urea cycle is TRUE?

• All steps occur exclusively in the mitochondrial matrix
• The cycle consumes two nitrogen atoms per molecule of urea formed
• Urea synthesis does not require ATP
• Argininosuccinate lyase converts arginine to ornithine

Correct Answer: The cycle consumes two nitrogen atoms per molecule of urea formed

Q39. Elevated blood levels of which amino acid are commonly used as a marker of liver dysfunction and impaired ammonia detoxification?

• Proline
• Glutamine
• Serine
• Tryptophan

Correct Answer: Glutamine

Q40. Which metabolic condition increases ureagenesis by raising N-acetylglutamate synthesis?

• Low-protein diet
• High-protein diet or increased amino acid catabolism
• Fasting ketosis
• Vitamin B6 deficiency

Correct Answer: High-protein diet or increased amino acid catabolism

Q41. Which amino acid carries nitrogen from peripheral tissues to the liver primarily via the glucose-alanine cycle?

• Glutamate
• Alanine
• Aspartate
• Leucine

Correct Answer: Alanine

Q42. Which inherited urea cycle defect often presents with episodic vomiting, lethargy, and coma in the neonatal period?

• Ornithine transcarbamylase (OTC) deficiency
• Hyperargininemia due to arginase deficiency
• Argininosuccinate lyase deficiency (ASL)
• Citrullinemia due to ASS deficiency

Correct Answer: Ornithine transcarbamylase (OTC) deficiency

Q43. Sodium phenylbutyrate helps treat hyperammonemia by converting to which compound that binds glutamine for excretion?

• Phenylacetate
• Benzoate
• Urea
• Ornithine

Correct Answer: Phenylacetate

Q44. Which enzyme deficiency leads to accumulation of argininosuccinate and citrulline, often causing mental retardation and liver dysfunction?

• Arginase deficiency
• Argininosuccinate lyase (ASL) deficiency
• Ornithine transcarbamylase deficiency
• Carbamoyl phosphate synthetase I deficiency

Correct Answer: Argininosuccinate lyase (ASL) deficiency

Q45. Which tissue has high activity of glutamine synthetase to mop up ammonia and protect the brain?

• Liver periportal hepatocytes
• Astrocytes in brain
• Skeletal muscle only
• Red blood cells

Correct Answer: Astrocytes in brain

Q46. Which of the following is a direct consequence of elevated systemic ammonia on neuronal metabolism?

• Decreased glutamine synthesis in astrocytes
• Excess glutamine accumulation causing osmotic swelling
• Increased urea production in brain neurons
• Enhanced oxidative phosphorylation

Correct Answer: Excess glutamine accumulation causing osmotic swelling

Q47. Which amino acid is the immediate precursor for synthesis of nitric oxide (NO) and is relevant to nitrogen metabolism?

• Arginine
• Glutamine
• Histidine
• Citrulline

Correct Answer: Arginine

Q48. Which pharmacologic measure reduces intestinal ammonia production by altering gut flora?

• Montelukast
• Rifaximin
• Proton pump inhibitors
• Loop diuretics

Correct Answer: Rifaximin

Q49. In the context of nitrogen metabolism, transdeamination refers to which combined process?

• Direct release of ammonia from amino acids without intermediates
• Transamination followed by oxidative deamination of glutamate
• Conversion of ammonia directly to urea in muscle
• Synthesis of new amino acids from CO2

Correct Answer: Transamination followed by oxidative deamination of glutamate

Q50. For B.Pharm students, why is understanding nitrogen metabolism clinically important?

• It has no practical relevance to pharmacology
• It links biochemical pathways to drug effects, toxicities, diagnostic tests, and treatment of metabolic disorders
• It only concerns plant physiology
• It is only relevant for nutritional counseling and not for drug therapy

Correct Answer: It links biochemical pathways to drug effects, toxicities, diagnostic tests, and treatment of metabolic disorders

G S Sachin

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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