WGU C785 Unit Exam Advanced Prep:
Master Biochemistry Practice Questions &
Detailed Explanations
Subject / Subtopic for Questions 1-30: Comprehensive Biochemistry (Biomedical
& Molecular Foundations)
Question 1: A 42-year-old female presents to the clinic complaining of severe muscle weakness,
recurrent cramping, and structural fatigue during moderate aerobic workouts. Muscle biopsy and
metabolic assays confirm a significant functional defect in the malate-aspartate shuttle system
across the inner mitochondrial membrane. Which of the following direct functional
consequences will be observed within this client's skeletal muscle cells during baseline cellular
respiration?
A) An accumulation of glycolytic NADH within the mitochondrial matrix.
B) A decreased capacity to transfer reducing equivalents from cytosolic NADH to the
mitochondrial electron transport chain.
C) Complete arrest of the citric acid cycle due to a lack of cytosolic oxaloacetate.
D) An increase in the direct generation of $FADH_2$ inside the intermembrane space.
Correct Answer: B) A decreased capacity to transfer reducing equivalents from cytosolic
NADH to the mitochondrial electron transport chain.
Explanation: The inner mitochondrial membrane is impermeable to NADH. The malate-
aspartate shuttle is a major structural system used to transfer reducing equivalents from
cytosolic NADH (generated during glycolysis) into the mitochondrial matrix so they can enter
Complex I of the electron transport chain. If this shuttle is defective, cytosolic NADH
accumulates, and the cell must rely heavily on anaerobic pathways (like lactate conversion) to
regenerate cytosolic $NAD^+$, reducing overall aerobic ATP yield and leading to fatigue and
muscle cramping. It does not cause mitochondrial matrix accumulation of NADH (Option A), as
the NADH cannot enter.
Question 2: An investigator analyzes a mutant mammalian cell line that displays an inability to
undergo normal DNA replication. Molecular analysis indicates that while the DNA double helix
unwinds correctly, the nascent lagging strand consists exclusively of short, fragmented segments
of DNA that fail to ligate into a continuous polymer. The functional deficiency is mapped to an
enzyme that catalyzes phosphodiester bond formation between adjacent Okazaki fragments.
Which of the following enzymes is mutated?
A) DNA Polymerase $\alpha$
,B) DNA Helicase
C) DNA Ligase
D) DNA Topoisomerase I
Correct Answer: C) DNA Ligase
Explanation: During DNA replication, the lagging strand is synthesized discontinuously as short
segments called Okazaki fragments. DNA Polymerase I (in prokaryotes) or DNA Polymerase
$\delta$/Flap endonuclease (in eukaryotes) removes the RNA primers and fills the gaps with
deoxyribonucleotides. However, a nick remains in the sugar-phosphate backbone. DNA ligase
specifically catalyzes the formation of a covalent phosphodiester bond between the $3'\text{-
hydroxyl}$ group of one fragment and the $5'\text{-phosphate}$ group of the adjacent fragment,
sealing the nick. A defect in DNA ligase results in fragmented, unlinked strands.
Question 3: A 3-week-old neonate is brought to the emergency department in a state of severe
lethargy, vomiting, and generalized hypertonicity. Plasma amino acid chromatography
demonstrates markedly elevated concentrations of the branched-chain amino acids leucine,
isoleucine, and valine, along with their corresponding $\alpha$-keto acid derivatives. Which of
the following structural conversions is directly impaired in this metabolic condition?
A) Transamination of amino acids to yield glutamate.
B) Oxidative decarboxylation of branched-chain $\alpha$-keto acids.
C) Hydroxylation of aromatic rings to synthesize tyrosine.
D) Interconversion of glycine and serine via tetrahydrofolate.
Correct Answer: B) Oxidative decarboxylation of branched-chain $\alpha$-keto acids.
Explanation: The presentation of neonatal metabolic distress paired with accumulated leucine,
isoleucine, and valine is diagnostic of Maple Syrup Urine Disease (MSUD). This autosomal
recessive disorder is caused by a structural or functional deficiency in the Branched-Chain
$\alpha$-Keto Acid Dehydrogenase complex. This complex is responsible for the oxidative
decarboxylation of the $\alpha$-keto acids derived from the transamination of branched-chain
amino acids. When this step is blocked, both the amino acids and their toxic keto acid
intermediates accumulate in the blood and urine, causing neurotoxicity.
Question 4: A research study examines the kinetic characteristics of an isolated human glycolytic
enzyme. Under physiological conditions, the reaction rate stabilizes at a specific velocity. When
a structural analog of the natural substrate is introduced into the reaction chamber, the apparent
Michaelis constant ($K_m$) of the enzyme increases significantly, while the maximum velocity
($V_{max}$) remains completely unchanged when the substrate concentration is elevated to
near-saturation. What type of enzyme inhibition is demonstrated?
, A) Non-competitive inhibition
B) Competitive inhibition
C) Uncompetitive inhibition
D) Irreversible suicide inhibition
Correct Answer: B) Competitive inhibition
Explanation: Competitive inhibitors are structural analogs that bind reversibly to the active site
of a free enzyme, competing directly with the natural substrate for access. Because they can be
displaced from the active site by adding an excess of the natural substrate, the maximum velocity
($V_{max}$) of the reaction remains unchanged. However, because higher concentrations of
substrate are required to achieve half-maximal velocity in the presence of the inhibitor, the
apparent Michaelis constant ($K_m$) increases.
Question 5: A 52-year-old client with a history of severe coronary artery disease is evaluated for
a suspected acute myocardial infarction. Within the ischemic cardiac myocytes, a profound
reduction in oxygen delivery forces the cells to shift from aerobic oxidative phosphorylation to
anaerobic glycolysis to generate ATP. Which of the following intracellular changes will directly
occur in these myocytes as a result of this metabolic shift?
A) An increase in intracellular pH due to the consumption of free protons.
B) Down-regulation of phosphofructokinase-1 (PFK-1) activity by high AMP levels.
C) Accelerated reduction of pyruvate to lactate by lactate dehydrogenase, resulting in
intracellular acidosis.
D) Increased production of acetyl-CoA within the mitochondrial matrix.
Correct Answer: C) Accelerated reduction of pyruvate to lactate by lactate dehydrogenase,
resulting in intracellular acidosis.
Explanation: In the absence of oxygen (hypoxia), the mitochondrial electron transport chain
stalls, causing an accumulation of NADH and a drop in ATP production. To generate ATP via
glycolysis, the cell must rapidly regenerate cytosolic $NAD^+$ from NADH. Lactate
dehydrogenase achieves this by reducing pyruvate into lactate, consuming NADH and releasing
$NAD^+$. The accumulation of lactic acid, alongside ATP hydrolysis, releases free protons
($H^+$), driving down intracellular pH (acidosis), which can ultimately cause lysosomal
enzyme activation and cell autolysis.
Question 6: A molecular biologist isolates a unique eukaryotic cell line that exhibits high rates of
errors during gene expression. Detailed sequence analysis reveals that the cells regularly produce
protein products with altered amino acid sequences despite having completely normal, mutation-
Master Biochemistry Practice Questions &
Detailed Explanations
Subject / Subtopic for Questions 1-30: Comprehensive Biochemistry (Biomedical
& Molecular Foundations)
Question 1: A 42-year-old female presents to the clinic complaining of severe muscle weakness,
recurrent cramping, and structural fatigue during moderate aerobic workouts. Muscle biopsy and
metabolic assays confirm a significant functional defect in the malate-aspartate shuttle system
across the inner mitochondrial membrane. Which of the following direct functional
consequences will be observed within this client's skeletal muscle cells during baseline cellular
respiration?
A) An accumulation of glycolytic NADH within the mitochondrial matrix.
B) A decreased capacity to transfer reducing equivalents from cytosolic NADH to the
mitochondrial electron transport chain.
C) Complete arrest of the citric acid cycle due to a lack of cytosolic oxaloacetate.
D) An increase in the direct generation of $FADH_2$ inside the intermembrane space.
Correct Answer: B) A decreased capacity to transfer reducing equivalents from cytosolic
NADH to the mitochondrial electron transport chain.
Explanation: The inner mitochondrial membrane is impermeable to NADH. The malate-
aspartate shuttle is a major structural system used to transfer reducing equivalents from
cytosolic NADH (generated during glycolysis) into the mitochondrial matrix so they can enter
Complex I of the electron transport chain. If this shuttle is defective, cytosolic NADH
accumulates, and the cell must rely heavily on anaerobic pathways (like lactate conversion) to
regenerate cytosolic $NAD^+$, reducing overall aerobic ATP yield and leading to fatigue and
muscle cramping. It does not cause mitochondrial matrix accumulation of NADH (Option A), as
the NADH cannot enter.
Question 2: An investigator analyzes a mutant mammalian cell line that displays an inability to
undergo normal DNA replication. Molecular analysis indicates that while the DNA double helix
unwinds correctly, the nascent lagging strand consists exclusively of short, fragmented segments
of DNA that fail to ligate into a continuous polymer. The functional deficiency is mapped to an
enzyme that catalyzes phosphodiester bond formation between adjacent Okazaki fragments.
Which of the following enzymes is mutated?
A) DNA Polymerase $\alpha$
,B) DNA Helicase
C) DNA Ligase
D) DNA Topoisomerase I
Correct Answer: C) DNA Ligase
Explanation: During DNA replication, the lagging strand is synthesized discontinuously as short
segments called Okazaki fragments. DNA Polymerase I (in prokaryotes) or DNA Polymerase
$\delta$/Flap endonuclease (in eukaryotes) removes the RNA primers and fills the gaps with
deoxyribonucleotides. However, a nick remains in the sugar-phosphate backbone. DNA ligase
specifically catalyzes the formation of a covalent phosphodiester bond between the $3'\text{-
hydroxyl}$ group of one fragment and the $5'\text{-phosphate}$ group of the adjacent fragment,
sealing the nick. A defect in DNA ligase results in fragmented, unlinked strands.
Question 3: A 3-week-old neonate is brought to the emergency department in a state of severe
lethargy, vomiting, and generalized hypertonicity. Plasma amino acid chromatography
demonstrates markedly elevated concentrations of the branched-chain amino acids leucine,
isoleucine, and valine, along with their corresponding $\alpha$-keto acid derivatives. Which of
the following structural conversions is directly impaired in this metabolic condition?
A) Transamination of amino acids to yield glutamate.
B) Oxidative decarboxylation of branched-chain $\alpha$-keto acids.
C) Hydroxylation of aromatic rings to synthesize tyrosine.
D) Interconversion of glycine and serine via tetrahydrofolate.
Correct Answer: B) Oxidative decarboxylation of branched-chain $\alpha$-keto acids.
Explanation: The presentation of neonatal metabolic distress paired with accumulated leucine,
isoleucine, and valine is diagnostic of Maple Syrup Urine Disease (MSUD). This autosomal
recessive disorder is caused by a structural or functional deficiency in the Branched-Chain
$\alpha$-Keto Acid Dehydrogenase complex. This complex is responsible for the oxidative
decarboxylation of the $\alpha$-keto acids derived from the transamination of branched-chain
amino acids. When this step is blocked, both the amino acids and their toxic keto acid
intermediates accumulate in the blood and urine, causing neurotoxicity.
Question 4: A research study examines the kinetic characteristics of an isolated human glycolytic
enzyme. Under physiological conditions, the reaction rate stabilizes at a specific velocity. When
a structural analog of the natural substrate is introduced into the reaction chamber, the apparent
Michaelis constant ($K_m$) of the enzyme increases significantly, while the maximum velocity
($V_{max}$) remains completely unchanged when the substrate concentration is elevated to
near-saturation. What type of enzyme inhibition is demonstrated?
, A) Non-competitive inhibition
B) Competitive inhibition
C) Uncompetitive inhibition
D) Irreversible suicide inhibition
Correct Answer: B) Competitive inhibition
Explanation: Competitive inhibitors are structural analogs that bind reversibly to the active site
of a free enzyme, competing directly with the natural substrate for access. Because they can be
displaced from the active site by adding an excess of the natural substrate, the maximum velocity
($V_{max}$) of the reaction remains unchanged. However, because higher concentrations of
substrate are required to achieve half-maximal velocity in the presence of the inhibitor, the
apparent Michaelis constant ($K_m$) increases.
Question 5: A 52-year-old client with a history of severe coronary artery disease is evaluated for
a suspected acute myocardial infarction. Within the ischemic cardiac myocytes, a profound
reduction in oxygen delivery forces the cells to shift from aerobic oxidative phosphorylation to
anaerobic glycolysis to generate ATP. Which of the following intracellular changes will directly
occur in these myocytes as a result of this metabolic shift?
A) An increase in intracellular pH due to the consumption of free protons.
B) Down-regulation of phosphofructokinase-1 (PFK-1) activity by high AMP levels.
C) Accelerated reduction of pyruvate to lactate by lactate dehydrogenase, resulting in
intracellular acidosis.
D) Increased production of acetyl-CoA within the mitochondrial matrix.
Correct Answer: C) Accelerated reduction of pyruvate to lactate by lactate dehydrogenase,
resulting in intracellular acidosis.
Explanation: In the absence of oxygen (hypoxia), the mitochondrial electron transport chain
stalls, causing an accumulation of NADH and a drop in ATP production. To generate ATP via
glycolysis, the cell must rapidly regenerate cytosolic $NAD^+$ from NADH. Lactate
dehydrogenase achieves this by reducing pyruvate into lactate, consuming NADH and releasing
$NAD^+$. The accumulation of lactic acid, alongside ATP hydrolysis, releases free protons
($H^+$), driving down intracellular pH (acidosis), which can ultimately cause lysosomal
enzyme activation and cell autolysis.
Question 6: A molecular biologist isolates a unique eukaryotic cell line that exhibits high rates of
errors during gene expression. Detailed sequence analysis reveals that the cells regularly produce
protein products with altered amino acid sequences despite having completely normal, mutation-
