NBME Comprehensive Basic Science 2

Question 1

A patient with chronic alcoholism presents with peripheral neuropathy and Wernicke's encephalopathy. Which enzyme requires the deficient cofactor?

Accepted Answer

Pyruvate dehydrogenase

Answer

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?

Accepted Answer

Facilitated diffusion via aquaporins

Answer

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?

Accepted Answer

DNA polymerase I

Answer

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?

Accepted Answer

Degradation of ubiquitin-tagged misfolded proteins

Answer

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?

Accepted Answer

Reduced translation initiation

Answer

The Shine-Dalgarno sequence base-pairs with 16S rRNA to position the ribosome at the start codon; its loss severely impairs translation initiation.

NBME Practice Test

NBME Practice Test

NBME Comprehensive Basic Science 2