WGU C785 Final Exam Advanced Prep:
Master Biochemistry Practice Questions &
Detailed Explanations
Subject / Subtopic for Questions 1-30: Comprehensive Biochemistry (Biomedical
& Molecular Foundations)
Question 1: A 4-year-old child presents with severe psychomotor delay, self-mutilating behavior
(lip and finger biting), and hyperuricemia. Laboratory tests reveal an accumulation of PRPP (5-
phosphoribosyl-1-pyrophosphate) and a near-total deficiency in an enzyme responsible for
purine salvage. Which of the following metabolic conversions is directly impaired due to this
specific enzyme deficiency?
A) Adenine to AMP
B) Hypoxanthine to IMP
C) Inosine to Hypoxanthine
D) Xanthine to Uric acid
Correct Answer: B) Hypoxanthine to IMP
Explanation: The clinical triad of self-mutilation, developmental delay, and hyperuricemia is
diagnostic of Lesch-Nyhan syndrome, an X-linked recessive disorder caused by a severe
deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT). HGPRT catalyzes the
salvage pathway conversion of hypoxanthine to IMP and guanine to GMP using PRPP as a co-
substrate. When HGPRT is deficient, these purines cannot be salvaged, leading to excessive
purine degradation into uric acid and an accumulation of PRPP, which drives de novo purine
synthesis. Conversion of adenine to AMP (Option A) is catalyzed by APRT, not HGPRT. Inosine
to hypoxanthine (Option C) is catalyzed by purine nucleoside phosphorylase, and xanthine to
uric acid (Option D) is mediated by xanthine oxidase.
Question 2: An investigator designs an in vitro assay to monitor structural changes in a cytosolic
globular protein. During the experiment, the buffer solution is inadvertently altered, causing a
gradual increase in temperature from 37°C to 55°C. Which of the following structural
disruptions will occur first during this thermal denaturation process?
A) Cleavage of peptide bonds within the core hydrophobic alpha-helices.
B) Disruption of non-covalent interactions stabilizing the tertiary structure.
C) Reduction of covalent disulfide bridges between adjacent cysteine residues.
,D) Complete dissociation of individual amino acids into a free form pool.
Correct Answer: B) Disruption of non-covalent interactions stabilizing the tertiary
structure.
Explanation: Thermal denaturation introduces kinetic energy into a protein system, which
disrupts the weak, non-covalent interactions—such as hydrogen bonds, hydrophobic
interactions, and van der Waals forces—that stabilize tertiary and secondary structures.
Covalent bonds, including peptide bonds (Option A) and disulfide bridges (Option C), possess
significantly higher bond energies and are structurally stable at 55°C in the absence of
enzymatic catalysts or heavy reducing agents. Individual amino acids are not broken apart into a
free pool (Option D) during thermal denaturation.
Question 3: A client presents with extreme fatigue, muscle weakness, and exercise intolerance. A
muscle biopsy reveals a significantly reduced capacity to transport long-chain fatty acids into the
mitochondrial matrix for $\beta$-oxidation. Analysis indicates a functional deficiency in
Carnitine Palmitoyltransferase I (CPT-1). Which of the following cellular molecules acts as the
physiological allosteric inhibitor of this specific enzyme?
A) Acetyl-CoA
B) Malonyl-CoA
C) Palmitoyl-CoA
D) Acetoacetate
Correct Answer: B) Malonyl-CoA
Explanation: $\beta$-oxidation and fatty acid synthesis are reciprocal pathways that are tightly
coordinated to prevent a futile cycle. Malonyl-CoA, the direct product of acetyl-CoA carboxylase
(the rate-limiting step of fatty acid synthesis), serves as a potent allosteric inhibitor of Carnitine
Palmitoyltransferase I (CPT-1). When fatty acid synthesis is active in the cytosol, high levels of
malonyl-CoA block CPT-1, preventing the entry of newly synthesized fatty acids into the
mitochondria for degradation. Acetyl-CoA (Option A), Palmitoyl-CoA (Option C), and
acetoacetate (Option D) do not function as the structural allosteric regulators of CPT-1 activity.
Question 4: A 6-month-old infant is brought to the emergency department experiencing severe
fasting hypoglycemia, hepatomegaly, and lactic acidosis. Physical examination confirms
significant growth restriction. Administration of epinephrine fails to induce an increase in blood
glucose levels, but glycogen structure on biopsy is found to be normal. A deficiency in which of
the following hepatic enzymes is most consistent with this presentation?
A) Glycogen phosphorylase
B) Glucose-6-phosphatase
,C) Phosphofructokinase-1
D) Pyruvate dehydrogenase
Correct Answer: B) Glucose-6-phosphatase
Explanation: The presentation of profound fasting hypoglycemia, lactic acidosis, hepatomegaly
(due to glycogen accumulation), and non-responsiveness to glycogenolytic hormones like
epinephrine is indicative of Von Gierke disease (Glycogen Storage Disease Type I), which is
caused by a deficiency in glucose-6-phosphatase. This enzyme catalyzes the final step of both
glycogenolysis and gluconeogenesis: the conversion of glucose-6-phosphate to free glucose.
Without it, the liver cannot export glucose into circulation. A deficiency in hepatic glycogen
phosphorylase (Option A, Hers disease) presents with milder hypoglycemia because
gluconeogenesis remains intact. PFK-1 (Option C) and Pyruvate dehydrogenase (Option D) are
glycolytic/oxidative enzymes whose deficiency does not match this clinical picture.
Question 5: A basic research study investigates the kinetic properties of a newly isolated hepatic
transaminase. The enzyme displays classic Michaelis-Menten kinetics. If the concentration of the
substrate is exactly equal to five times the Michaelis constant ($K_m$), what fraction of the
maximum velocity ($V_{max}$) will the reaction rate ($v_0$) be?
A) $\frac{1}{5} V_{max}$
B) $\frac{1}{6} V_{max}$
C) $\frac{5}{6} V_{max}$
D) $\frac{4}{5} V_{max}$
Correct Answer: C) $\frac{5}{6} V_{max}$
Explanation: To find the reaction rate relative to $V_{max}$, use the Michaelis-Menten
equation: $v_0 = \frac{V_{max}[S]}{K_m + [S]}$. Given that $[S] = 5K_m$, substitute this
value into the equation: $v_0 = \frac{V_{max}(5K_m)}{K_m + 5K_m} =
\frac{5V_{max}K_m}{6K_m}$. Canceling out $K_m$ yields $v_0 = \frac{5}{6} V_{max}$.
Therefore, the reaction rate will operate at exactly $\frac{5}{6}$ (or approximately 83.3%) of its
maximum possible velocity.
Question 6: A 24-year-old individual traveling in a malaria-endemic region is prescribed
primaquine for prophylaxis. Three days later, the individual develops dark urine, jaundice, and
severe hemolytic anemia. Laboratory evaluation reveals Heinz bodies within the erythrocytes.
This clinical reaction is caused by an inability to regenerate which of the following biochemical
cofactors?
A) NADH
, B) $FADH_2$
C) NADPH
D) TPP
Correct Answer: C) NADPH
Explanation: The individual has Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency, an X-
linked recessive disorder that impairs the first step of the pentose phosphate pathway.
Erythrocytes rely exclusively on this pathway to generate NADPH, which is mandatory to
maintain glutathione in its reduced state. Reduced glutathione detoxifies reactive oxygen species
(ROS) induced by oxidative stressors like primaquine, fava beans, or infections. Without
NADPH, glutathione remains oxidized, leading to oxidative damage to hemoglobin, cross-
linking sulfhydryl groups, and creating insoluble precipitates known as Heinz bodies, which
trigger intravascular hemolysis.
Question 7: A molecular biologist analyzes the replication of DNA in a prokaryotic system. The
researcher blocks the function of an enzyme that possesses a unique $5' \to 3'$ exonuclease
activity used to remove RNA primers and replace them with deoxyribonucleotides. Which of the
following enzymes has been targeted?
A) DNA Polymerase III
B) DNA Polymerase I
C) DNA Helicase
D) DNA Topoisomerase II
Correct Answer: B) DNA Polymerase I
Explanation: In prokaryotic DNA replication, DNA Polymerase I plays a critical role in lagging
strand maturation due to its unique $5' \to 3'$ exonuclease activity. This allows the enzyme to
excise the ribonucleotide primers (RNA primers) ahead of it while simultaneously synthesizing
complementary DNA using its $5' \to 3'$ polymerase activity (a process known as nick
translation). DNA Polymerase III (Option A) is the primary replicative enzyme and possesses a
$3' \to 5'$ proofreading exonuclease activity, but lacks the $5' \to 3'$ exonuclease capability.
Helicase (Option C) unwinds DNA, and Topoisomerase II (Option D) relieves supercoiling.
Question 8: A 45-year-old chronic alcoholic presents with confusion, nystagmus, and severe
ataxia. The physician suspects Wernicke encephalopathy and administers intravenous thiamine
immediately. Thiamine pyrophosphate (TPP) serves as an essential cofactor for which of the
following enzymes involved in cellular energy metabolism?
A) Isocitrate dehydrogenase
Master Biochemistry Practice Questions &
Detailed Explanations
Subject / Subtopic for Questions 1-30: Comprehensive Biochemistry (Biomedical
& Molecular Foundations)
Question 1: A 4-year-old child presents with severe psychomotor delay, self-mutilating behavior
(lip and finger biting), and hyperuricemia. Laboratory tests reveal an accumulation of PRPP (5-
phosphoribosyl-1-pyrophosphate) and a near-total deficiency in an enzyme responsible for
purine salvage. Which of the following metabolic conversions is directly impaired due to this
specific enzyme deficiency?
A) Adenine to AMP
B) Hypoxanthine to IMP
C) Inosine to Hypoxanthine
D) Xanthine to Uric acid
Correct Answer: B) Hypoxanthine to IMP
Explanation: The clinical triad of self-mutilation, developmental delay, and hyperuricemia is
diagnostic of Lesch-Nyhan syndrome, an X-linked recessive disorder caused by a severe
deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT). HGPRT catalyzes the
salvage pathway conversion of hypoxanthine to IMP and guanine to GMP using PRPP as a co-
substrate. When HGPRT is deficient, these purines cannot be salvaged, leading to excessive
purine degradation into uric acid and an accumulation of PRPP, which drives de novo purine
synthesis. Conversion of adenine to AMP (Option A) is catalyzed by APRT, not HGPRT. Inosine
to hypoxanthine (Option C) is catalyzed by purine nucleoside phosphorylase, and xanthine to
uric acid (Option D) is mediated by xanthine oxidase.
Question 2: An investigator designs an in vitro assay to monitor structural changes in a cytosolic
globular protein. During the experiment, the buffer solution is inadvertently altered, causing a
gradual increase in temperature from 37°C to 55°C. Which of the following structural
disruptions will occur first during this thermal denaturation process?
A) Cleavage of peptide bonds within the core hydrophobic alpha-helices.
B) Disruption of non-covalent interactions stabilizing the tertiary structure.
C) Reduction of covalent disulfide bridges between adjacent cysteine residues.
,D) Complete dissociation of individual amino acids into a free form pool.
Correct Answer: B) Disruption of non-covalent interactions stabilizing the tertiary
structure.
Explanation: Thermal denaturation introduces kinetic energy into a protein system, which
disrupts the weak, non-covalent interactions—such as hydrogen bonds, hydrophobic
interactions, and van der Waals forces—that stabilize tertiary and secondary structures.
Covalent bonds, including peptide bonds (Option A) and disulfide bridges (Option C), possess
significantly higher bond energies and are structurally stable at 55°C in the absence of
enzymatic catalysts or heavy reducing agents. Individual amino acids are not broken apart into a
free pool (Option D) during thermal denaturation.
Question 3: A client presents with extreme fatigue, muscle weakness, and exercise intolerance. A
muscle biopsy reveals a significantly reduced capacity to transport long-chain fatty acids into the
mitochondrial matrix for $\beta$-oxidation. Analysis indicates a functional deficiency in
Carnitine Palmitoyltransferase I (CPT-1). Which of the following cellular molecules acts as the
physiological allosteric inhibitor of this specific enzyme?
A) Acetyl-CoA
B) Malonyl-CoA
C) Palmitoyl-CoA
D) Acetoacetate
Correct Answer: B) Malonyl-CoA
Explanation: $\beta$-oxidation and fatty acid synthesis are reciprocal pathways that are tightly
coordinated to prevent a futile cycle. Malonyl-CoA, the direct product of acetyl-CoA carboxylase
(the rate-limiting step of fatty acid synthesis), serves as a potent allosteric inhibitor of Carnitine
Palmitoyltransferase I (CPT-1). When fatty acid synthesis is active in the cytosol, high levels of
malonyl-CoA block CPT-1, preventing the entry of newly synthesized fatty acids into the
mitochondria for degradation. Acetyl-CoA (Option A), Palmitoyl-CoA (Option C), and
acetoacetate (Option D) do not function as the structural allosteric regulators of CPT-1 activity.
Question 4: A 6-month-old infant is brought to the emergency department experiencing severe
fasting hypoglycemia, hepatomegaly, and lactic acidosis. Physical examination confirms
significant growth restriction. Administration of epinephrine fails to induce an increase in blood
glucose levels, but glycogen structure on biopsy is found to be normal. A deficiency in which of
the following hepatic enzymes is most consistent with this presentation?
A) Glycogen phosphorylase
B) Glucose-6-phosphatase
,C) Phosphofructokinase-1
D) Pyruvate dehydrogenase
Correct Answer: B) Glucose-6-phosphatase
Explanation: The presentation of profound fasting hypoglycemia, lactic acidosis, hepatomegaly
(due to glycogen accumulation), and non-responsiveness to glycogenolytic hormones like
epinephrine is indicative of Von Gierke disease (Glycogen Storage Disease Type I), which is
caused by a deficiency in glucose-6-phosphatase. This enzyme catalyzes the final step of both
glycogenolysis and gluconeogenesis: the conversion of glucose-6-phosphate to free glucose.
Without it, the liver cannot export glucose into circulation. A deficiency in hepatic glycogen
phosphorylase (Option A, Hers disease) presents with milder hypoglycemia because
gluconeogenesis remains intact. PFK-1 (Option C) and Pyruvate dehydrogenase (Option D) are
glycolytic/oxidative enzymes whose deficiency does not match this clinical picture.
Question 5: A basic research study investigates the kinetic properties of a newly isolated hepatic
transaminase. The enzyme displays classic Michaelis-Menten kinetics. If the concentration of the
substrate is exactly equal to five times the Michaelis constant ($K_m$), what fraction of the
maximum velocity ($V_{max}$) will the reaction rate ($v_0$) be?
A) $\frac{1}{5} V_{max}$
B) $\frac{1}{6} V_{max}$
C) $\frac{5}{6} V_{max}$
D) $\frac{4}{5} V_{max}$
Correct Answer: C) $\frac{5}{6} V_{max}$
Explanation: To find the reaction rate relative to $V_{max}$, use the Michaelis-Menten
equation: $v_0 = \frac{V_{max}[S]}{K_m + [S]}$. Given that $[S] = 5K_m$, substitute this
value into the equation: $v_0 = \frac{V_{max}(5K_m)}{K_m + 5K_m} =
\frac{5V_{max}K_m}{6K_m}$. Canceling out $K_m$ yields $v_0 = \frac{5}{6} V_{max}$.
Therefore, the reaction rate will operate at exactly $\frac{5}{6}$ (or approximately 83.3%) of its
maximum possible velocity.
Question 6: A 24-year-old individual traveling in a malaria-endemic region is prescribed
primaquine for prophylaxis. Three days later, the individual develops dark urine, jaundice, and
severe hemolytic anemia. Laboratory evaluation reveals Heinz bodies within the erythrocytes.
This clinical reaction is caused by an inability to regenerate which of the following biochemical
cofactors?
A) NADH
, B) $FADH_2$
C) NADPH
D) TPP
Correct Answer: C) NADPH
Explanation: The individual has Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency, an X-
linked recessive disorder that impairs the first step of the pentose phosphate pathway.
Erythrocytes rely exclusively on this pathway to generate NADPH, which is mandatory to
maintain glutathione in its reduced state. Reduced glutathione detoxifies reactive oxygen species
(ROS) induced by oxidative stressors like primaquine, fava beans, or infections. Without
NADPH, glutathione remains oxidized, leading to oxidative damage to hemoglobin, cross-
linking sulfhydryl groups, and creating insoluble precipitates known as Heinz bodies, which
trigger intravascular hemolysis.
Question 7: A molecular biologist analyzes the replication of DNA in a prokaryotic system. The
researcher blocks the function of an enzyme that possesses a unique $5' \to 3'$ exonuclease
activity used to remove RNA primers and replace them with deoxyribonucleotides. Which of the
following enzymes has been targeted?
A) DNA Polymerase III
B) DNA Polymerase I
C) DNA Helicase
D) DNA Topoisomerase II
Correct Answer: B) DNA Polymerase I
Explanation: In prokaryotic DNA replication, DNA Polymerase I plays a critical role in lagging
strand maturation due to its unique $5' \to 3'$ exonuclease activity. This allows the enzyme to
excise the ribonucleotide primers (RNA primers) ahead of it while simultaneously synthesizing
complementary DNA using its $5' \to 3'$ polymerase activity (a process known as nick
translation). DNA Polymerase III (Option A) is the primary replicative enzyme and possesses a
$3' \to 5'$ proofreading exonuclease activity, but lacks the $5' \to 3'$ exonuclease capability.
Helicase (Option C) unwinds DNA, and Topoisomerase II (Option D) relieves supercoiling.
Question 8: A 45-year-old chronic alcoholic presents with confusion, nystagmus, and severe
ataxia. The physician suspects Wernicke encephalopathy and administers intravenous thiamine
immediately. Thiamine pyrophosphate (TPP) serves as an essential cofactor for which of the
following enzymes involved in cellular energy metabolism?
A) Isocitrate dehydrogenase
