NBME Comprehensive Basic Science 2 — Questions and Answers
Question 1: A patient with chronic alcoholism presents with peripheral neuropathy and Wernicke's encephalopathy. Which enzyme requires the deficient cofactor?
- Pyruvate dehydrogenase (Correct answer)
- Lactate dehydrogenase
- Glucose-6-phosphate dehydrogenase
- Glutamate dehydrogenase
Correct answer: Pyruvate dehydrogenase
Thiamine (B1) is required by pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase; its deficiency impairs oxidative decarboxylation.
Question 2: A cell exposed to hypotonic solution swells and lyses. Which membrane transport mechanism is responsible for the initial water influx?
- Facilitated diffusion via aquaporins (Correct answer)
- Active transport via Na+/K+ ATPase
- Vesicle-mediated endocytosis
- Carrier-mediated secondary active transport
Correct answer: Facilitated diffusion via aquaporins
Water moves down its osmotic gradient through aquaporin channels by facilitated diffusion, not requiring ATP.
Question 3: During DNA replication, which enzyme removes RNA primers and fills the resulting gaps with DNA?
- DNA polymerase I (Correct answer)
- DNA polymerase III
- DNA ligase
- Primase
Correct answer: DNA polymerase I
In prokaryotes, DNA polymerase I has 5'→3' exonuclease activity to remove primers and 5'→3' polymerase activity to fill gaps.
Question 4: A researcher blocks the proteasome in a cell. Which of the following processes is most directly impaired?
- Degradation of ubiquitin-tagged misfolded proteins (Correct answer)
- Lysosomal autophagy of organelles
- Translation of mRNA by ribosomes
- Post-translational glycosylation in the Golgi
Correct answer: Degradation of ubiquitin-tagged misfolded proteins
The proteasome degrades cytosolic proteins tagged with polyubiquitin chains, including misfolded and regulatory proteins.
Question 5: A mutation eliminates the Shine-Dalgarno sequence in a bacterial mRNA. What is the most likely result?
- Reduced translation initiation (Correct answer)
- Premature transcription termination
- Inability to splice introns
- Failure of mRNA capping
Correct answer: Reduced translation initiation
The Shine-Dalgarno sequence base-pairs with 16S rRNA to position the ribosome at the start codon; its loss severely impairs translation initiation.
Question 6: Which glycolytic intermediate directly donates phosphate to ADP to produce ATP via substrate-level phosphorylation?
- Phosphoenolpyruvate (Correct answer)
- Fructose-1,6-bisphosphate
- Glucose-6-phosphate
- 3-Phosphoglycerate
Correct answer: Phosphoenolpyruvate
Pyruvate kinase transfers the phosphate from phosphoenolpyruvate to ADP, generating pyruvate and ATP by substrate-level phosphorylation.
Question 7: A patient has a defect in the enzyme that hydroxylates proline residues in collagen. Which cofactor is most likely deficient?
- Ascorbic acid (Vitamin C) (Correct answer)
- Pyridoxal phosphate (B6)
- Cobalamin (B12)
- Riboflavin (B2)
Correct answer: Ascorbic acid (Vitamin C)
Prolyl hydroxylase requires Vitamin C as a cofactor; deficiency leads to scurvy with defective collagen cross-linking.
NBME Comprehensive Basic Science 2
A patient with chronic alcoholism presents with peripheral neuropathy and Wernicke's encephalopathy.
Which enzyme requires the deficient cofactor?