MCAT BIOLOGICAL & BIOCHEMICAL FOUNDATIONS,
CHEMICAL & PHYSICAL FOUNDATIONS PSYCHOLOGICAL,
SOCIAL & BIOLOGICAL FOUNDATIONS, CRITICAL
ANALYSIS & REASONING SKILLS EXAM QUESTIONS AND
CORRECT ANSWERS WITH RATIONALES GRADED A+
LATEST
BIO/BIOCHEM, CHEM/PHYS, PSYCH/SOC, CARS EXAM
Section 1 — Biological & Biochemical Foundations
Passage: A researcher isolates a bacterial strain that grows only when provided
with a compound X. Growth ceases when compound X is removed. The researcher
suspects compound X is an essential cofactor for an enzyme in central metabolism.
Q1. Which experimental observation would best support that compound X is a
cofactor rather than a substrate for the enzyme?
A. Compound X concentration remains constant during enzymatic reaction.
B. Compound X is chemically consumed and converted into product.
C. Enzyme activity increases when substrate concentration increases.
D. Compound X binds irreversibly to enzyme active site.
Answer: A
Rationale: A cofactor is not consumed during catalysis and often remains
unchanged (or is regenerated), so its concentration stays constant. If it were a
substrate it would be consumed (B). Irreversible binding (D) would inactivate
enzyme; C is about substrate kinetics.
Q2. A mutation reduces the affinity of hemoglobin for 2,3-bisphosphoglycerate
(2,3-BPG). Which physiological effect is expected?
A. Increased oxygen unloading in tissues.
B. Right shift of the oxygen–hemoglobin dissociation curve.
,C. Increased hemoglobin oxygen affinity and decreased tissue oxygen delivery.
D. Decreased arterial oxygen saturation.
Answer: C
Rationale: Reduced 2,3-BPG binding increases Hb affinity for O₂, shifting curve
left, decreasing unloading to tissues and thus reducing tissue oxygen delivery.
Right shift and increased unloading are opposite.
Q3. An enzyme follows Michaelis–Menten kinetics. Vmax is 100 µmol/min and
Km is 5 mM. At what substrate concentration is reaction velocity 50 µmol/min?
A. 2.5 mM
B. 5 mM
C. 10 mM
D. 20 mM
Answer: B
Rationale: At [S] = Km, velocity = Vmax/2 (Michaelis–Menten). Thus 5 mM.
Q4. A protein is synthesized and then glycosylated in the ER. A drug blocks N-
linked glycosylation. Which consequence is most likely?
A. Protein cannot enter the Golgi.
B. Protein will accumulate misfolded in the ER and be targeted for ER-associated
degradation (ERAD).
C. Protein will be secreted more rapidly.
D. Protein will be targeted to mitochondria.
Answer: B
Rationale: N-linked glycosylation assists folding and quality control; blocking it
often causes misfolding and ERAD. Golgi entry (A) may be reduced but not the
primary effect; secretion faster (C) unlikely.
,Q5. Which amino acid change is most likely to disrupt an α-helix in a globular
protein?
A. Leucine → Valine
B. Alanine → Glycine
C. Glutamate → Aspartate
D. Lysine → Arginine
Answer: B
Rationale: Glycine is helix-destabilizing due to conformational flexibility; alanine
favors helices. Other substitutions are conservative.
Q6. During fasting, which process in the liver increases to provide glucose for
brain and red blood cells?
A. Glycogen synthesis
B. Gluconeogenesis
C. Glycolysis
D. Lipogenesis
Answer: B
Rationale: During fasting, gluconeogenesis increases to synthesize glucose from
noncarbohydrate precursors. Glycogen stores are used initially but gluconeogenesis
is key for prolonged fasting.
Q7. A patient has a deficiency in pyruvate dehydrogenase complex (PDH). Which
metabolic change is expected?
A. Increased acetyl-CoA production from pyruvate
B. Increased lactate production
C. Enhanced entry of carbons into TCA cycle from glucose
D. Decreased pyruvate levels
Answer: B
Rationale: PDH deficiency blocks conversion of pyruvate to acetyl-CoA, shunting
pyruvate to lactate via lactate dehydrogenase, increasing lactate.
, Q8. Which DNA repair mechanism corrects thymine dimers caused by UV light?
A. Base excision repair
B. Nucleotide excision repair
C. Non-homologous end joining
D. Mismatch repair
Answer: B
Rationale: Nucleotide excision repair removes bulky lesions like thymine dimers;
BER corrects small base modifications; NHEJ repairs double-strand breaks.
Q9. In oxidative phosphorylation, which complex pumps protons from matrix to
intermembrane space?
A. Complex II only
B. Complex IV only
C. Complex I, III, and IV
D. ATP synthase
Answer: C
Rationale: Complexes I, III, and IV pump protons; Complex II does not. ATP
synthase uses proton flow back into matrix to produce ATP.
Q10. A ligand increases the rate at which a receptor is internalized and degraded,
reducing receptor number on the surface. This phenomenon is best described as:
A. Receptor sensitization
B. Receptor down-regulation
C. Desensitization by phosphorylation
D. Competitive antagonism
Answer: B
Rationale: Down-regulation reduces receptor numbers via
internalization/degradation. Desensitization often refers to decreased signaling
despite receptor presence.
11. Q11. A patient has a point mutation in a tRNA gene that reduces
aminoacylation. Which cellular process is directly impaired?
CHEMICAL & PHYSICAL FOUNDATIONS PSYCHOLOGICAL,
SOCIAL & BIOLOGICAL FOUNDATIONS, CRITICAL
ANALYSIS & REASONING SKILLS EXAM QUESTIONS AND
CORRECT ANSWERS WITH RATIONALES GRADED A+
LATEST
BIO/BIOCHEM, CHEM/PHYS, PSYCH/SOC, CARS EXAM
Section 1 — Biological & Biochemical Foundations
Passage: A researcher isolates a bacterial strain that grows only when provided
with a compound X. Growth ceases when compound X is removed. The researcher
suspects compound X is an essential cofactor for an enzyme in central metabolism.
Q1. Which experimental observation would best support that compound X is a
cofactor rather than a substrate for the enzyme?
A. Compound X concentration remains constant during enzymatic reaction.
B. Compound X is chemically consumed and converted into product.
C. Enzyme activity increases when substrate concentration increases.
D. Compound X binds irreversibly to enzyme active site.
Answer: A
Rationale: A cofactor is not consumed during catalysis and often remains
unchanged (or is regenerated), so its concentration stays constant. If it were a
substrate it would be consumed (B). Irreversible binding (D) would inactivate
enzyme; C is about substrate kinetics.
Q2. A mutation reduces the affinity of hemoglobin for 2,3-bisphosphoglycerate
(2,3-BPG). Which physiological effect is expected?
A. Increased oxygen unloading in tissues.
B. Right shift of the oxygen–hemoglobin dissociation curve.
,C. Increased hemoglobin oxygen affinity and decreased tissue oxygen delivery.
D. Decreased arterial oxygen saturation.
Answer: C
Rationale: Reduced 2,3-BPG binding increases Hb affinity for O₂, shifting curve
left, decreasing unloading to tissues and thus reducing tissue oxygen delivery.
Right shift and increased unloading are opposite.
Q3. An enzyme follows Michaelis–Menten kinetics. Vmax is 100 µmol/min and
Km is 5 mM. At what substrate concentration is reaction velocity 50 µmol/min?
A. 2.5 mM
B. 5 mM
C. 10 mM
D. 20 mM
Answer: B
Rationale: At [S] = Km, velocity = Vmax/2 (Michaelis–Menten). Thus 5 mM.
Q4. A protein is synthesized and then glycosylated in the ER. A drug blocks N-
linked glycosylation. Which consequence is most likely?
A. Protein cannot enter the Golgi.
B. Protein will accumulate misfolded in the ER and be targeted for ER-associated
degradation (ERAD).
C. Protein will be secreted more rapidly.
D. Protein will be targeted to mitochondria.
Answer: B
Rationale: N-linked glycosylation assists folding and quality control; blocking it
often causes misfolding and ERAD. Golgi entry (A) may be reduced but not the
primary effect; secretion faster (C) unlikely.
,Q5. Which amino acid change is most likely to disrupt an α-helix in a globular
protein?
A. Leucine → Valine
B. Alanine → Glycine
C. Glutamate → Aspartate
D. Lysine → Arginine
Answer: B
Rationale: Glycine is helix-destabilizing due to conformational flexibility; alanine
favors helices. Other substitutions are conservative.
Q6. During fasting, which process in the liver increases to provide glucose for
brain and red blood cells?
A. Glycogen synthesis
B. Gluconeogenesis
C. Glycolysis
D. Lipogenesis
Answer: B
Rationale: During fasting, gluconeogenesis increases to synthesize glucose from
noncarbohydrate precursors. Glycogen stores are used initially but gluconeogenesis
is key for prolonged fasting.
Q7. A patient has a deficiency in pyruvate dehydrogenase complex (PDH). Which
metabolic change is expected?
A. Increased acetyl-CoA production from pyruvate
B. Increased lactate production
C. Enhanced entry of carbons into TCA cycle from glucose
D. Decreased pyruvate levels
Answer: B
Rationale: PDH deficiency blocks conversion of pyruvate to acetyl-CoA, shunting
pyruvate to lactate via lactate dehydrogenase, increasing lactate.
, Q8. Which DNA repair mechanism corrects thymine dimers caused by UV light?
A. Base excision repair
B. Nucleotide excision repair
C. Non-homologous end joining
D. Mismatch repair
Answer: B
Rationale: Nucleotide excision repair removes bulky lesions like thymine dimers;
BER corrects small base modifications; NHEJ repairs double-strand breaks.
Q9. In oxidative phosphorylation, which complex pumps protons from matrix to
intermembrane space?
A. Complex II only
B. Complex IV only
C. Complex I, III, and IV
D. ATP synthase
Answer: C
Rationale: Complexes I, III, and IV pump protons; Complex II does not. ATP
synthase uses proton flow back into matrix to produce ATP.
Q10. A ligand increases the rate at which a receptor is internalized and degraded,
reducing receptor number on the surface. This phenomenon is best described as:
A. Receptor sensitization
B. Receptor down-regulation
C. Desensitization by phosphorylation
D. Competitive antagonism
Answer: B
Rationale: Down-regulation reduces receptor numbers via
internalization/degradation. Desensitization often refers to decreased signaling
despite receptor presence.
11. Q11. A patient has a point mutation in a tRNA gene that reduces
aminoacylation. Which cellular process is directly impaired?
