## **Core Concept**
The cell cycle is regulated by a family of proteins known as cyclins, which bind to and activate cyclin-dependent kinases (CDKs). These cyclin-CDK complexes drive the progression through the cell cycle phases, including G1, S, G2, and M. The transition from the G2 phase to the M phase is crucial for cell division.

## **Why the Correct Answer is Right**
The correct answer, **Cyclin B**, is right because Cyclin B binds to and activates CDK1 (also known as CDC2), forming the M-phase promoting factor (MPF). This complex is essential for the G2 to M phase transition. The activation of Cyclin B-CDK1 complex leads to the phosphorylation of various substrates that induce the breakdown of the nuclear envelope, condensation of chromosomes, and the initiation of mitosis.

## **Why Each Wrong Option is Incorrect**
- **Option A: Cyclin A** - While Cyclin A is involved in the cell cycle, particularly in the S phase and the G2 phase, it is not the primary cyclin responsible for the G2 to M phase transition.
- **Option B: Cyclin D** - Cyclin D is crucial for the G1 to S phase transition by binding to and activating CDK4 or CDK6, which phosphorylate and inactivate the retinoblastoma protein (pRb), allowing the cell to progress through the G1 phase.
- **Option D: Cyclin E** - Cyclin E, which binds to CDK2, is important for the G1 to S phase transition. It helps in the phosphorylation of pRb, similar to Cyclin D, but at a later stage.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the G2 to M phase transition is a critical checkpoint in the cell cycle. Dysregulation of this process can lead to uncontrolled cell division, a hallmark of cancer. Drugs that target the cell cycle, including those that inhibit CDKs, are being explored for cancer therapy.

## **Correct Answer: C. Cyclin B**