BMSC510 Medical Biochemistry Practice Questions and Answers Updated
2026 | Complete Biochemistry Study Guide with Verified Questions,
Detailed Rationales, Biomolecules, Enzymes, Metabolism Pathways,
Carbohydrate, Lipid & Protein Metabolism, DNA/RNA Structure, Molecular
Biology, Clinical Biochemistry & Biomedical Science Exam Prep
Question 1: Which amino acid serves as the precursor for the synthesis of nitric
oxide, a critical signaling molecule in vascular endothelial function?
A. Lysine
B. Arginine
C. Citrulline
D. Ornithine
CORRECT ANSWER: B. Arginine
Rationale: Nitric oxide (NO) is synthesized from L-arginine by nitric oxide synthase
(NOS) enzymes, producing NO and citrulline as products. This reaction is essential for
vasodilation, neurotransmission, and immune response. While citrulline can be
recycled to arginine, arginine is the direct substrate for NO production.
Question 2: In the glycolytic pathway, which enzyme catalyzes the committed, rate-
limiting step that is allosterically inhibited by ATP and citrate?
A. Hexokinase
B. Phosphofructokinase-1
C. Pyruvate kinase
D. Aldolase
CORRECT ANSWER: B. Phosphofructokinase-1
Rationale: Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-
phosphate to fructose-1,6-bisphosphate, the first irreversible step unique to glycolysis.
PFK-1 is the primary regulatory enzyme of glycolysis, inhibited by high energy signals
(ATP, citrate) and activated by AMP and fructose-2,6-bisphosphate.
Question 3: Which of the following best describes the primary biochemical defect
in phenylketonuria (PKU)?
A. Deficiency of homogentisate oxidase
B. Deficiency of phenylalanine hydroxylase
C. Deficiency of branched-chain α-ketoacid dehydrogenase
D. Deficiency of cystathionine β-synthase
CORRECT ANSWER: B. Deficiency of phenylalanine hydroxylase
Rationale: PKU results from mutations in the gene encoding phenylalanine hydroxylase,
which converts phenylalanine to tyrosine. Accumulation of phenylalanine and its
metabolites causes neurotoxicity. Early dietary restriction of phenylalanine prevents
intellectual disability.
,BMSC510 Medical Biochemistry Practice Questions and Answers Updated
2026 | Complete Biochemistry Study Guide with Verified Questions,
Detailed Rationales, Biomolecules, Enzymes, Metabolism Pathways,
Carbohydrate, Lipid & Protein Metabolism, DNA/RNA Structure, Molecular
Biology, Clinical Biochemistry & Biomedical Science Exam Prep
Question 4: During β-oxidation of fatty acids, which coenzyme is reduced in the
first dehydrogenation step catalyzed by acyl-CoA dehydrogenase?
A. NAD⁺
B. NADP⁺
C. FAD
D. Coenzyme Q
CORRECT ANSWER: C. FAD
Rationale: Acyl-CoA dehydrogenase catalyzes the introduction of a trans-double bond
between C2 and C3 of acyl-CoA, reducing FAD to FADH₂. This is the first step in each β-
oxidation cycle. FADH₂ subsequently donates electrons to the electron transport chain
via electron transfer flavoprotein.
Question 5: Which vitamin-derived cofactor is essential for the transamination
reactions that interconvert amino acids and α-keto acids?
A. Thiamine pyrophosphate (TPP)
B. Pyridoxal phosphate (PLP)
C. Flavin adenine dinucleotide (FAD)
D. Tetrahydrofolate (THF)
CORRECT ANSWER: B. Pyridoxal phosphate (PLP)
Rationale: Pyridoxal phosphate, derived from vitamin B₆, serves as the coenzyme for
aminotransferases (transaminases). PLP forms a Schiff base with amino acids,
facilitating the transfer of amino groups to α-keto acids, a central process in amino acid
metabolism and nitrogen balance.
Question 6: In the urea cycle, which compound is synthesized in the mitochondria
and transported to the cytosol for subsequent reactions?
A. Ornithine
B. Citrulline
C. Argininosuccinate
D. Fumarate
CORRECT ANSWER: B. Citrulline
Rationale: Carbamoyl phosphate and ornithine condense in the mitochondrial matrix to
form citrulline via ornithine transcarbamylase. Citrulline is then transported to the
cytosol, where it combines with aspartate to form argininosuccinate. This
compartmentalization is critical for urea cycle regulation.
,BMSC510 Medical Biochemistry Practice Questions and Answers Updated
2026 | Complete Biochemistry Study Guide with Verified Questions,
Detailed Rationales, Biomolecules, Enzymes, Metabolism Pathways,
Carbohydrate, Lipid & Protein Metabolism, DNA/RNA Structure, Molecular
Biology, Clinical Biochemistry & Biomedical Science Exam Prep
Question 7: Which of the following molecules acts as a key allosteric activator of
pyruvate carboxylase, linking glycolysis to gluconeogenesis?
A. ATP
B. Acetyl-CoA
C. Citrate
D. Fructose-2,6-bisphosphate
CORRECT ANSWER: B. Acetyl-CoA
Rationale: Pyruvate carboxylase, which converts pyruvate to oxaloacetate, is absolutely
dependent on acetyl-CoA as an allosteric activator. High acetyl-CoA levels signal
abundant fuel for the TCA cycle, promoting gluconeogenesis to maintain blood glucose
during fasting.
Question 8: The pentose phosphate pathway primarily generates which two critical
products for cellular metabolism?
A. ATP and NADH
B. NADPH and ribose-5-phosphate
C. FADH₂ and glucose-6-phosphate
D. GTP and erythrose-4-phosphate
CORRECT ANSWER: B. NADPH and ribose-5-phosphate
Rationale: The oxidative phase of the pentose phosphate pathway produces NADPH
(essential for reductive biosynthesis and antioxidant defense) and ribose-5-phosphate
(a precursor for nucleotide and nucleic acid synthesis). The non-oxidative phase
interconverts sugar phosphates to meet metabolic demands.
Question 9: Which enzyme deficiency causes Tay-Sachs disease, characterized by
accumulation of GM2 ganglioside in neuronal lysosomes?
A. Glucocerebrosidase
B. Hexosaminidase A
C. Sphingomyelinase
D. α-Galactosidase A
CORRECT ANSWER: B. Hexosaminidase A
Rationale: Tay-Sachs disease results from mutations in the HEXA gene encoding the α-
subunit of hexosaminidase A. Deficiency impairs degradation of GM2 ganglioside,
leading to progressive neurodegeneration, cherry-red spot on macula, and early
childhood mortality.
, BMSC510 Medical Biochemistry Practice Questions and Answers Updated
2026 | Complete Biochemistry Study Guide with Verified Questions,
Detailed Rationales, Biomolecules, Enzymes, Metabolism Pathways,
Carbohydrate, Lipid & Protein Metabolism, DNA/RNA Structure, Molecular
Biology, Clinical Biochemistry & Biomedical Science Exam Prep
Question 10: In oxidative phosphorylation, which complex of the electron transport
chain does NOT pump protons across the inner mitochondrial membrane?
A. Complex I (NADH dehydrogenase)
B. Complex II (succinate dehydrogenase)
C. Complex III (cytochrome bc₁ complex)
D. Complex IV (cytochrome c oxidase)
CORRECT ANSWER: B. Complex II (succinate dehydrogenase)
Rationale: Complex II transfers electrons from succinate to ubiquinone but does not
contribute to the proton gradient because it lacks proton-pumping capability.
Complexes I, III, and IV actively pump protons, establishing the electrochemical
gradient used by ATP synthase.
Question 11: Which of the following best describes the biochemical role of biotin in
carboxylation reactions?
A. Electron carrier in redox reactions
B. Carrier of activated CO₂ groups
C. Methyl group donor in methylation reactions
D. Acyl group carrier in fatty acid synthesis
CORRECT ANSWER: B. Carrier of activated CO₂ groups
Rationale: Biotin serves as a covalently bound cofactor for carboxylases (e.g., pyruvate
carboxylase, acetyl-CoA carboxylase). It carries activated bicarbonate (CO₂) as
carboxybiotin, facilitating the addition of carboxyl groups to substrates in
gluconeogenesis and fatty acid synthesis.
Question 12: During glycogen synthesis, which enzyme catalyzes the formation of
α-1,4-glycosidic bonds using UDP-glucose as the glucose donor?
A. Glycogen phosphorylase
B. Glycogen synthase
C. Branching enzyme
D. Phosphoglucomutase
CORRECT ANSWER: B. Glycogen synthase
Rationale: Glycogen synthase transfers glucose from UDP-glucose to the non-reducing
end of a growing glycogen chain, forming α-1,4-glycosidic linkages. This enzyme is
regulated by covalent modification (phosphorylation inactivates) and allosteric
effectors (glucose-6-phosphate activates).
2026 | Complete Biochemistry Study Guide with Verified Questions,
Detailed Rationales, Biomolecules, Enzymes, Metabolism Pathways,
Carbohydrate, Lipid & Protein Metabolism, DNA/RNA Structure, Molecular
Biology, Clinical Biochemistry & Biomedical Science Exam Prep
Question 1: Which amino acid serves as the precursor for the synthesis of nitric
oxide, a critical signaling molecule in vascular endothelial function?
A. Lysine
B. Arginine
C. Citrulline
D. Ornithine
CORRECT ANSWER: B. Arginine
Rationale: Nitric oxide (NO) is synthesized from L-arginine by nitric oxide synthase
(NOS) enzymes, producing NO and citrulline as products. This reaction is essential for
vasodilation, neurotransmission, and immune response. While citrulline can be
recycled to arginine, arginine is the direct substrate for NO production.
Question 2: In the glycolytic pathway, which enzyme catalyzes the committed, rate-
limiting step that is allosterically inhibited by ATP and citrate?
A. Hexokinase
B. Phosphofructokinase-1
C. Pyruvate kinase
D. Aldolase
CORRECT ANSWER: B. Phosphofructokinase-1
Rationale: Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-
phosphate to fructose-1,6-bisphosphate, the first irreversible step unique to glycolysis.
PFK-1 is the primary regulatory enzyme of glycolysis, inhibited by high energy signals
(ATP, citrate) and activated by AMP and fructose-2,6-bisphosphate.
Question 3: Which of the following best describes the primary biochemical defect
in phenylketonuria (PKU)?
A. Deficiency of homogentisate oxidase
B. Deficiency of phenylalanine hydroxylase
C. Deficiency of branched-chain α-ketoacid dehydrogenase
D. Deficiency of cystathionine β-synthase
CORRECT ANSWER: B. Deficiency of phenylalanine hydroxylase
Rationale: PKU results from mutations in the gene encoding phenylalanine hydroxylase,
which converts phenylalanine to tyrosine. Accumulation of phenylalanine and its
metabolites causes neurotoxicity. Early dietary restriction of phenylalanine prevents
intellectual disability.
,BMSC510 Medical Biochemistry Practice Questions and Answers Updated
2026 | Complete Biochemistry Study Guide with Verified Questions,
Detailed Rationales, Biomolecules, Enzymes, Metabolism Pathways,
Carbohydrate, Lipid & Protein Metabolism, DNA/RNA Structure, Molecular
Biology, Clinical Biochemistry & Biomedical Science Exam Prep
Question 4: During β-oxidation of fatty acids, which coenzyme is reduced in the
first dehydrogenation step catalyzed by acyl-CoA dehydrogenase?
A. NAD⁺
B. NADP⁺
C. FAD
D. Coenzyme Q
CORRECT ANSWER: C. FAD
Rationale: Acyl-CoA dehydrogenase catalyzes the introduction of a trans-double bond
between C2 and C3 of acyl-CoA, reducing FAD to FADH₂. This is the first step in each β-
oxidation cycle. FADH₂ subsequently donates electrons to the electron transport chain
via electron transfer flavoprotein.
Question 5: Which vitamin-derived cofactor is essential for the transamination
reactions that interconvert amino acids and α-keto acids?
A. Thiamine pyrophosphate (TPP)
B. Pyridoxal phosphate (PLP)
C. Flavin adenine dinucleotide (FAD)
D. Tetrahydrofolate (THF)
CORRECT ANSWER: B. Pyridoxal phosphate (PLP)
Rationale: Pyridoxal phosphate, derived from vitamin B₆, serves as the coenzyme for
aminotransferases (transaminases). PLP forms a Schiff base with amino acids,
facilitating the transfer of amino groups to α-keto acids, a central process in amino acid
metabolism and nitrogen balance.
Question 6: In the urea cycle, which compound is synthesized in the mitochondria
and transported to the cytosol for subsequent reactions?
A. Ornithine
B. Citrulline
C. Argininosuccinate
D. Fumarate
CORRECT ANSWER: B. Citrulline
Rationale: Carbamoyl phosphate and ornithine condense in the mitochondrial matrix to
form citrulline via ornithine transcarbamylase. Citrulline is then transported to the
cytosol, where it combines with aspartate to form argininosuccinate. This
compartmentalization is critical for urea cycle regulation.
,BMSC510 Medical Biochemistry Practice Questions and Answers Updated
2026 | Complete Biochemistry Study Guide with Verified Questions,
Detailed Rationales, Biomolecules, Enzymes, Metabolism Pathways,
Carbohydrate, Lipid & Protein Metabolism, DNA/RNA Structure, Molecular
Biology, Clinical Biochemistry & Biomedical Science Exam Prep
Question 7: Which of the following molecules acts as a key allosteric activator of
pyruvate carboxylase, linking glycolysis to gluconeogenesis?
A. ATP
B. Acetyl-CoA
C. Citrate
D. Fructose-2,6-bisphosphate
CORRECT ANSWER: B. Acetyl-CoA
Rationale: Pyruvate carboxylase, which converts pyruvate to oxaloacetate, is absolutely
dependent on acetyl-CoA as an allosteric activator. High acetyl-CoA levels signal
abundant fuel for the TCA cycle, promoting gluconeogenesis to maintain blood glucose
during fasting.
Question 8: The pentose phosphate pathway primarily generates which two critical
products for cellular metabolism?
A. ATP and NADH
B. NADPH and ribose-5-phosphate
C. FADH₂ and glucose-6-phosphate
D. GTP and erythrose-4-phosphate
CORRECT ANSWER: B. NADPH and ribose-5-phosphate
Rationale: The oxidative phase of the pentose phosphate pathway produces NADPH
(essential for reductive biosynthesis and antioxidant defense) and ribose-5-phosphate
(a precursor for nucleotide and nucleic acid synthesis). The non-oxidative phase
interconverts sugar phosphates to meet metabolic demands.
Question 9: Which enzyme deficiency causes Tay-Sachs disease, characterized by
accumulation of GM2 ganglioside in neuronal lysosomes?
A. Glucocerebrosidase
B. Hexosaminidase A
C. Sphingomyelinase
D. α-Galactosidase A
CORRECT ANSWER: B. Hexosaminidase A
Rationale: Tay-Sachs disease results from mutations in the HEXA gene encoding the α-
subunit of hexosaminidase A. Deficiency impairs degradation of GM2 ganglioside,
leading to progressive neurodegeneration, cherry-red spot on macula, and early
childhood mortality.
, BMSC510 Medical Biochemistry Practice Questions and Answers Updated
2026 | Complete Biochemistry Study Guide with Verified Questions,
Detailed Rationales, Biomolecules, Enzymes, Metabolism Pathways,
Carbohydrate, Lipid & Protein Metabolism, DNA/RNA Structure, Molecular
Biology, Clinical Biochemistry & Biomedical Science Exam Prep
Question 10: In oxidative phosphorylation, which complex of the electron transport
chain does NOT pump protons across the inner mitochondrial membrane?
A. Complex I (NADH dehydrogenase)
B. Complex II (succinate dehydrogenase)
C. Complex III (cytochrome bc₁ complex)
D. Complex IV (cytochrome c oxidase)
CORRECT ANSWER: B. Complex II (succinate dehydrogenase)
Rationale: Complex II transfers electrons from succinate to ubiquinone but does not
contribute to the proton gradient because it lacks proton-pumping capability.
Complexes I, III, and IV actively pump protons, establishing the electrochemical
gradient used by ATP synthase.
Question 11: Which of the following best describes the biochemical role of biotin in
carboxylation reactions?
A. Electron carrier in redox reactions
B. Carrier of activated CO₂ groups
C. Methyl group donor in methylation reactions
D. Acyl group carrier in fatty acid synthesis
CORRECT ANSWER: B. Carrier of activated CO₂ groups
Rationale: Biotin serves as a covalently bound cofactor for carboxylases (e.g., pyruvate
carboxylase, acetyl-CoA carboxylase). It carries activated bicarbonate (CO₂) as
carboxybiotin, facilitating the addition of carboxyl groups to substrates in
gluconeogenesis and fatty acid synthesis.
Question 12: During glycogen synthesis, which enzyme catalyzes the formation of
α-1,4-glycosidic bonds using UDP-glucose as the glucose donor?
A. Glycogen phosphorylase
B. Glycogen synthase
C. Branching enzyme
D. Phosphoglucomutase
CORRECT ANSWER: B. Glycogen synthase
Rationale: Glycogen synthase transfers glucose from UDP-glucose to the non-reducing
end of a growing glycogen chain, forming α-1,4-glycosidic linkages. This enzyme is
regulated by covalent modification (phosphorylation inactivates) and allosteric
effectors (glucose-6-phosphate activates).
