Which of the following is the radiosensitive stage of cell cycle?

**Core Concept**
The radiosensitive stage of the cell cycle refers to the phase where cells are most vulnerable to damage from ionizing radiation. This is crucial in understanding the effects of radiation therapy in cancer treatment and the potential side effects on rapidly dividing cells.

**Why the Correct Answer is Right**
The G2 phase is the radiosensitive stage of the cell cycle. During this phase, cells are preparing for mitosis and have a high concentration of DNA, making them more susceptible to radiation-induced damage. This is because DNA replication and repair mechanisms are less active in G2, leaving cells more vulnerable to radiation-induced DNA damage. The G2 checkpoint ensures that any DNA damage is repaired before the cell proceeds to mitosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** The G1 phase is not considered radiosensitive because cells have not yet replicated their DNA, and DNA repair mechanisms are more active.
* **Option B:** The S phase is not radiosensitive because cells are actively replicating their DNA, and DNA repair mechanisms are highly active during this phase.
* **Option C:** The M phase is not radiosensitive because cells are undergoing mitosis, and the DNA is condensed, making it less susceptible to radiation-induced damage.

**Clinical Pearl / High-Yield Fact**
Radiosensitivity is a critical factor in radiation oncology, and understanding the cell cycle is essential in designing effective radiation therapy protocols.

**Correct Answer:** C. G2 phase.