, Question 1
A mutation in the gene encoding the enzyme pyruvate dehydrogenase kinase
(PDK) results in a constitutively active kinase. Which metabolic consequence
is most likely in a well-oxygenated hepatocyte?
A. Increased conversion of pyruvate to acetyl-CoA
B. Enhanced flux through the citric acid cycle
C. Impaired conversion of pyruvate to acetyl-CoA
D. Stimulation of gluconeogenesis from lactate
Correct Answer: C - Impaired conversion of pyruvate to
acetyl-CoA
RATIONALE
PDK phosphorylates and inhibits pyruvate dehydrogenase (PDH), the
enzyme that converts pyruvate to acetyl-CoA. Constitutive PDK
activity would keep PDH inhibited even under aerobic conditions,
reducing acetyl-CoA production. This would also limit citric acid
cycle flux and promote alternative fates of pyruvate such as
gluconeogenesis, but the direct and immediate consequence is
impaired acetyl-CoA formation.
Question 2
In a population of Drosophila, a recessive X-linked lethal allele causes death in
hemizygous males before eclosion. If the allele frequency is 0.1 in females,
what proportion of adult females will be carriers?
A. 0.09
B. 0.18
C. 0.42
D. 0.90
Correct Answer: B - 0.18
Page 2
, RATIONALE
For an X-linked recessive lethal, adult males do not carry the allele.
Assuming Hardy-Weinberg equilibrium among adult females, let q be
the frequency of the lethal allele among X chromosomes in females.
Given q = 0.1, the frequency of heterozygous carriers is 2pq =
2(0.9)(0.1) = 0.18. Homozygous recessive females (q² = 0.01) may
also die or be affected, but the question asks for carriers.
Question 3
Which observation best supports the endosymbiotic theory for the origin of
mitochondria?
A. Mitochondria contain circular DNA and divide by binary fission.
B. Mitochondria are enclosed by a double membrane.
C. Mitochondria synthesize some proteins using their own ribosomes.
D. Mitochondria are inherited maternally in most eukaryotes.
Correct Answer: A - Mitochondria contain circular DNA and
divide by binary fission.
RATIONALE
The presence of circular DNA and binary fission is a strong indicator
of bacterial ancestry, as these features are characteristic of
prokaryotes. While double membranes and independent protein
synthesis also support endosymbiosis, circular DNA and division by
fission are more direct evidence of a prokaryotic origin.
Question 4
A patient with a mutation in the gene encoding the -subunit of the epithelial
sodium channel (ENaC) exhibits salt wasting and hypotension. Which renal
tubule segment is most directly affected?
A. Proximal convoluted tubule
B. Thick ascending limb of the loop of Henle
C. Distal convoluted tubule
Page 3
, D. Collecting duct
Correct Answer: D - Collecting duct
RATIONALE
ENaC is primarily located in the apical membrane of principal cells in
the collecting duct, where it mediates sodium reabsorption under the
regulation of aldosterone. Mutations causing loss of function lead to
salt wasting and hypotension. The other segments have different
sodium transport mechanisms.
Question 5
During an experiment, a researcher adds a drug that inhibits the enzyme
dihydrofolate reductase (DHFR) to a culture of rapidly dividing cells. Which
process will be most immediately impaired?
A. DNA replication
B. Transcription
C. Translation
D. Glycolysis
Correct Answer: A - DNA replication
RATIONALE
DHFR is essential for the synthesis of tetrahydrofolate, which is
required for the de novo synthesis of thymidylate and purines,
precursors for DNA replication. Inhibition rapidly halts DNA
synthesis. Transcription and translation are less directly affected, and
glycolysis does not require folate.
Question 6
Which of the following best explains why the theoretical maximum ATP yield
from complete oxidation of one glucose molecule is lower than the often-cited
36-38 ATP?
A. Some ATP is consumed during the transport of NADH into
Page 4
A mutation in the gene encoding the enzyme pyruvate dehydrogenase kinase
(PDK) results in a constitutively active kinase. Which metabolic consequence
is most likely in a well-oxygenated hepatocyte?
A. Increased conversion of pyruvate to acetyl-CoA
B. Enhanced flux through the citric acid cycle
C. Impaired conversion of pyruvate to acetyl-CoA
D. Stimulation of gluconeogenesis from lactate
Correct Answer: C - Impaired conversion of pyruvate to
acetyl-CoA
RATIONALE
PDK phosphorylates and inhibits pyruvate dehydrogenase (PDH), the
enzyme that converts pyruvate to acetyl-CoA. Constitutive PDK
activity would keep PDH inhibited even under aerobic conditions,
reducing acetyl-CoA production. This would also limit citric acid
cycle flux and promote alternative fates of pyruvate such as
gluconeogenesis, but the direct and immediate consequence is
impaired acetyl-CoA formation.
Question 2
In a population of Drosophila, a recessive X-linked lethal allele causes death in
hemizygous males before eclosion. If the allele frequency is 0.1 in females,
what proportion of adult females will be carriers?
A. 0.09
B. 0.18
C. 0.42
D. 0.90
Correct Answer: B - 0.18
Page 2
, RATIONALE
For an X-linked recessive lethal, adult males do not carry the allele.
Assuming Hardy-Weinberg equilibrium among adult females, let q be
the frequency of the lethal allele among X chromosomes in females.
Given q = 0.1, the frequency of heterozygous carriers is 2pq =
2(0.9)(0.1) = 0.18. Homozygous recessive females (q² = 0.01) may
also die or be affected, but the question asks for carriers.
Question 3
Which observation best supports the endosymbiotic theory for the origin of
mitochondria?
A. Mitochondria contain circular DNA and divide by binary fission.
B. Mitochondria are enclosed by a double membrane.
C. Mitochondria synthesize some proteins using their own ribosomes.
D. Mitochondria are inherited maternally in most eukaryotes.
Correct Answer: A - Mitochondria contain circular DNA and
divide by binary fission.
RATIONALE
The presence of circular DNA and binary fission is a strong indicator
of bacterial ancestry, as these features are characteristic of
prokaryotes. While double membranes and independent protein
synthesis also support endosymbiosis, circular DNA and division by
fission are more direct evidence of a prokaryotic origin.
Question 4
A patient with a mutation in the gene encoding the -subunit of the epithelial
sodium channel (ENaC) exhibits salt wasting and hypotension. Which renal
tubule segment is most directly affected?
A. Proximal convoluted tubule
B. Thick ascending limb of the loop of Henle
C. Distal convoluted tubule
Page 3
, D. Collecting duct
Correct Answer: D - Collecting duct
RATIONALE
ENaC is primarily located in the apical membrane of principal cells in
the collecting duct, where it mediates sodium reabsorption under the
regulation of aldosterone. Mutations causing loss of function lead to
salt wasting and hypotension. The other segments have different
sodium transport mechanisms.
Question 5
During an experiment, a researcher adds a drug that inhibits the enzyme
dihydrofolate reductase (DHFR) to a culture of rapidly dividing cells. Which
process will be most immediately impaired?
A. DNA replication
B. Transcription
C. Translation
D. Glycolysis
Correct Answer: A - DNA replication
RATIONALE
DHFR is essential for the synthesis of tetrahydrofolate, which is
required for the de novo synthesis of thymidylate and purines,
precursors for DNA replication. Inhibition rapidly halts DNA
synthesis. Transcription and translation are less directly affected, and
glycolysis does not require folate.
Question 6
Which of the following best explains why the theoretical maximum ATP yield
from complete oxidation of one glucose molecule is lower than the often-cited
36-38 ATP?
A. Some ATP is consumed during the transport of NADH into
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