General physiology: cell cycle: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · General physiology: cell cycle

Sample item

A woman with breast cancer is given a CDK4/6 inhibitor. At which point of the cell cycle does the drug arrest cells?

AThe G2 to M transition, because cyclin B-CDK1 cannot be activatedBThe G1 to S transition, because retinoblastoma protein stays unphosphorylated and holds E2FCThe metaphase to anaphase transition, because the spindle checkpoint is activatedDWithin S phase, because DNA polymerase is directly blocked

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. The G1 to S transition, because retinoblastoma protein stays unphosphorylated and holds E2F

Cyclin D with CDK4/6 phosphorylates retinoblastoma protein in G1, releasing E2F to drive transcription of S-phase genes. Inhibiting CDK4/6 keeps the protein hypophosphorylated and cells stop at the G1 restriction point. G2/M passage depends on cyclin B-CDK1 and the spindle checkpoint acts later.

Option by option

A. The G2 to M transition, because cyclin B-CDK1 cannot be activated
Cyclin B-CDK1 controls entry into mitosis and is not the target of CDK4/6 inhibitors.
B. The G1 to S transition, because retinoblastoma protein stays unphosphorylated and holds E2F (correct)
Correct. Retinoblastoma protein phosphorylation is the gate at the G1 to S restriction point.
C. The metaphase to anaphase transition, because the spindle checkpoint is activated
The spindle assembly checkpoint operates at metaphase and is not engaged by this drug.
D. Within S phase, because DNA polymerase is directly blocked
DNA polymerase inhibition is the action of nucleoside analogues, not CDK4/6 inhibitors.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is B, The G1 to S transition, because retinoblastoma protein stays unphosphorylated and holds E2F. Cyclin D with CDK4/6 phosphorylates retinoblastoma protein in G1, releasing E2F to drive transcription of S-phase genes.

Why is "The G2 to M transition, because cyclin B-CDK1 cannot be activated" not the answer?

Cyclin B-CDK1 controls entry into mitosis and is not the target of CDK4/6 inhibitors.

Why is "The metaphase to anaphase transition, because the spindle checkpoint…" not the answer?

The spindle assembly checkpoint operates at metaphase and is not engaged by this drug.

Why is "Within S phase, because DNA polymerase is directly blocked" not the answer?

DNA polymerase inhibition is the action of nucleoside analogues, not CDK4/6 inhibitors.

What concept does this question test?

It tests General physiology: cell cycle in Physiology. Inhibiting CDK4/6 keeps the protein hypophosphorylated and cells stop at the G1 restriction point. G2/M passage depends on cyclin B-CDK1 and the spindle checkpoint acts later.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General physiology: cell cycle?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.