Medical College Admission Test (MCAT) –
Advanced Practice Question Bank 150 Multiple-
Choice Questions with Correct Answers and
Rationales a well detailed one written
and graded A+ upgraded
SECTION 1: BIOLOGICAL AND BIOCHEMICAL FOUNDATIONS OF LIVING SYSTEMS (Questions 1–
40)
Question 1
A researcher isolates a novel enzyme that catalyzes the conversion of substrate S to product P.
She observes that the reaction velocity reaches half of Vmax at a substrate concentration of 25
μM. If she adds a competitive inhibitor at a concentration equal to Ki, what substrate
concentration would be required to achieve the same reaction velocity?
A) 12.5 μM
B) 25 μM
C) 50 μM
D) 100 μM
Correct Answer: C) 50 μM
Rationale: In competitive inhibition, the apparent Km increases by a factor of (1 + [I]/Ki). When
[I] = Ki, the apparent Km doubles from 25 μM to 50 μM. To achieve the same velocity (which
was half of Vmax without inhibitor), the substrate concentration must equal the new apparent
Km.
,Question 2
Which of the following metabolic pathways would be MOST directly affected by a deficiency in
thiamine pyrophosphate (TPP)?
A) Glycolysis only
B) The citric acid cycle only
C) Both glycolysis and the citric acid cycle
D) Fatty acid β-oxidation only
Correct Answer: C) Both glycolysis and the citric acid cycle
Rationale: TPP is a coenzyme for pyruvate dehydrogenase (linking glycolysis to the TCA cycle)
and α-ketoglutarate dehydrogenase (in the TCA cycle). It is also required for transketolase in the
pentose phosphate pathway. Deficiency would impair both glycolysis (via pyruvate
dehydrogenase) and the citric acid cycle (via α-ketoglutarate dehydrogenase).
Question 3
A patient presents with elevated blood levels of ornithine, citrulline, and argininosuccinate but
low levels of urea. Which enzyme is most likely deficient?
A) Carbamoyl phosphate synthetase I
B) Ornithine transcarbamoylase
C) Argininosuccinate synthase
D) Arginase
Correct Answer: C) Argininosuccinate synthase
Rationale: Argininosuccinate synthase catalyzes the condensation of citrulline and aspartate to
form argininosuccinate. Deficiency of this enzyme causes citrullinemia, characterized by
elevated citrulline and argininosuccinate, with impaired urea production.
Question 4
Which of the following statements about the lac operon is CORRECT?
A) The operator sequence binds RNA polymerase directly
B) CAP-cAMP binding enhances transcription when glucose is abundant
C) The repressor protein binds to the operator in the absence of allolactose
D) Structural genes are transcribed continuously at low levels regardless of conditions
Correct Answer: C) The repressor protein binds to the operator in the absence of allolactose
,Rationale: The lac repressor binds to the operator sequence when allolactose (the inducer) is
absent, blocking transcription. CAP-cAMP binds when glucose is scarce (not abundant) to
enhance transcription. RNA polymerase binds the promoter, not the operator.
Question 5
A mutation in the gene encoding telomerase reverse transcriptase (TERT) would most directly
affect which cellular process?
A) DNA replication during mitosis
B) The maintenance of chromosome ends in germ cells
C) Transcription of ribosomal RNA
D) mRNA splicing
Correct Answer: B) The maintenance of chromosome ends in germ cells
Rationale: Telomerase adds repetitive nucleotide sequences to chromosome telomeres,
preventing progressive shortening during DNA replication. Telomerase is active in germ cells,
stem cells, and cancer cells. Somatic cells typically lack telomerase activity.
Question 6
In a resting neuron, the membrane potential is approximately -70 mV. Which of the following
contributes LEAST to the establishment of this resting membrane potential?
A) The Na⁺/K⁺ ATPase pump
B) Potassium leak channels
C) Sodium leak channels
D) Voltage-gated calcium channels
Correct Answer: D) Voltage-gated calcium channels
Rationale: Voltage-gated calcium channels are generally closed at resting membrane potential.
The resting potential is primarily established by the Na⁺/K⁺ ATPase (maintaining concentration
gradients), potassium leak channels (allowing K⁺ efflux), and to a lesser extent sodium leak
channels.
Question 7
Which amino acid substitution in a protein's active site would most likely abolish enzymatic
activity without significantly altering protein folding?
, A) Valine to leucine
B) Aspartate to asparagine
C) Lysine to arginine
D) Serine to threonine
Correct Answer: B) Aspartate to asparagine
Rationale: Aspartate (negatively charged) to asparagine (neutral) changes the charge at the
active site, likely disrupting catalytic function. Valine→leucine, lysine→arginine, and
serine→threonine are conservative substitutions that maintain similar chemical properties.
Question 8
A 35-year-old male presents with progressive muscle weakness, exercise intolerance, and
elevated serum creatine kinase. A muscle biopsy reveals glycogen accumulation. Which enzyme
deficiency is most consistent with these findings?
A) Glucose-6-phosphatase
B) Myophosphorylase (muscle glycogen phosphorylase)
C) Liver glycogen phosphorylase
D) Phosphoglucomutase
Correct Answer: B) Myophosphorylase (muscle glycogen phosphorylase)
Rationale: McArdle disease results from myophosphorylase deficiency, causing inability to
break down muscle glycogen. Symptoms include exercise intolerance, muscle cramping, and
elevated creatine kinase. Glycogen accumulates in muscle tissue.
Question 9
The synthesis of a polypeptide chain on the rough endoplasmic reticulum requires all of the
following EXCEPT:
A) Signal recognition particle (SRP)
B) SRP receptor
C) A stop-transfer sequence
D) A nuclear localization signal
Correct Answer: D) A nuclear localization signal
Advanced Practice Question Bank 150 Multiple-
Choice Questions with Correct Answers and
Rationales a well detailed one written
and graded A+ upgraded
SECTION 1: BIOLOGICAL AND BIOCHEMICAL FOUNDATIONS OF LIVING SYSTEMS (Questions 1–
40)
Question 1
A researcher isolates a novel enzyme that catalyzes the conversion of substrate S to product P.
She observes that the reaction velocity reaches half of Vmax at a substrate concentration of 25
μM. If she adds a competitive inhibitor at a concentration equal to Ki, what substrate
concentration would be required to achieve the same reaction velocity?
A) 12.5 μM
B) 25 μM
C) 50 μM
D) 100 μM
Correct Answer: C) 50 μM
Rationale: In competitive inhibition, the apparent Km increases by a factor of (1 + [I]/Ki). When
[I] = Ki, the apparent Km doubles from 25 μM to 50 μM. To achieve the same velocity (which
was half of Vmax without inhibitor), the substrate concentration must equal the new apparent
Km.
,Question 2
Which of the following metabolic pathways would be MOST directly affected by a deficiency in
thiamine pyrophosphate (TPP)?
A) Glycolysis only
B) The citric acid cycle only
C) Both glycolysis and the citric acid cycle
D) Fatty acid β-oxidation only
Correct Answer: C) Both glycolysis and the citric acid cycle
Rationale: TPP is a coenzyme for pyruvate dehydrogenase (linking glycolysis to the TCA cycle)
and α-ketoglutarate dehydrogenase (in the TCA cycle). It is also required for transketolase in the
pentose phosphate pathway. Deficiency would impair both glycolysis (via pyruvate
dehydrogenase) and the citric acid cycle (via α-ketoglutarate dehydrogenase).
Question 3
A patient presents with elevated blood levels of ornithine, citrulline, and argininosuccinate but
low levels of urea. Which enzyme is most likely deficient?
A) Carbamoyl phosphate synthetase I
B) Ornithine transcarbamoylase
C) Argininosuccinate synthase
D) Arginase
Correct Answer: C) Argininosuccinate synthase
Rationale: Argininosuccinate synthase catalyzes the condensation of citrulline and aspartate to
form argininosuccinate. Deficiency of this enzyme causes citrullinemia, characterized by
elevated citrulline and argininosuccinate, with impaired urea production.
Question 4
Which of the following statements about the lac operon is CORRECT?
A) The operator sequence binds RNA polymerase directly
B) CAP-cAMP binding enhances transcription when glucose is abundant
C) The repressor protein binds to the operator in the absence of allolactose
D) Structural genes are transcribed continuously at low levels regardless of conditions
Correct Answer: C) The repressor protein binds to the operator in the absence of allolactose
,Rationale: The lac repressor binds to the operator sequence when allolactose (the inducer) is
absent, blocking transcription. CAP-cAMP binds when glucose is scarce (not abundant) to
enhance transcription. RNA polymerase binds the promoter, not the operator.
Question 5
A mutation in the gene encoding telomerase reverse transcriptase (TERT) would most directly
affect which cellular process?
A) DNA replication during mitosis
B) The maintenance of chromosome ends in germ cells
C) Transcription of ribosomal RNA
D) mRNA splicing
Correct Answer: B) The maintenance of chromosome ends in germ cells
Rationale: Telomerase adds repetitive nucleotide sequences to chromosome telomeres,
preventing progressive shortening during DNA replication. Telomerase is active in germ cells,
stem cells, and cancer cells. Somatic cells typically lack telomerase activity.
Question 6
In a resting neuron, the membrane potential is approximately -70 mV. Which of the following
contributes LEAST to the establishment of this resting membrane potential?
A) The Na⁺/K⁺ ATPase pump
B) Potassium leak channels
C) Sodium leak channels
D) Voltage-gated calcium channels
Correct Answer: D) Voltage-gated calcium channels
Rationale: Voltage-gated calcium channels are generally closed at resting membrane potential.
The resting potential is primarily established by the Na⁺/K⁺ ATPase (maintaining concentration
gradients), potassium leak channels (allowing K⁺ efflux), and to a lesser extent sodium leak
channels.
Question 7
Which amino acid substitution in a protein's active site would most likely abolish enzymatic
activity without significantly altering protein folding?
, A) Valine to leucine
B) Aspartate to asparagine
C) Lysine to arginine
D) Serine to threonine
Correct Answer: B) Aspartate to asparagine
Rationale: Aspartate (negatively charged) to asparagine (neutral) changes the charge at the
active site, likely disrupting catalytic function. Valine→leucine, lysine→arginine, and
serine→threonine are conservative substitutions that maintain similar chemical properties.
Question 8
A 35-year-old male presents with progressive muscle weakness, exercise intolerance, and
elevated serum creatine kinase. A muscle biopsy reveals glycogen accumulation. Which enzyme
deficiency is most consistent with these findings?
A) Glucose-6-phosphatase
B) Myophosphorylase (muscle glycogen phosphorylase)
C) Liver glycogen phosphorylase
D) Phosphoglucomutase
Correct Answer: B) Myophosphorylase (muscle glycogen phosphorylase)
Rationale: McArdle disease results from myophosphorylase deficiency, causing inability to
break down muscle glycogen. Symptoms include exercise intolerance, muscle cramping, and
elevated creatine kinase. Glycogen accumulates in muscle tissue.
Question 9
The synthesis of a polypeptide chain on the rough endoplasmic reticulum requires all of the
following EXCEPT:
A) Signal recognition particle (SRP)
B) SRP receptor
C) A stop-transfer sequence
D) A nuclear localization signal
Correct Answer: D) A nuclear localization signal
