**Core Concept**
The cell cycle is tightly regulated by a complex interplay of checkpoints and signaling pathways that ensure genomic integrity and prevent cancer. DNA damage triggers a halt in cell cycle progression to allow for repair or apoptosis. The key gene responsible for mediating this response is a crucial regulator of the cell cycle.

**Why the Correct Answer is Right**
The correct answer is p53, a tumor suppressor protein that acts as a guardian of the genome. When DNA damage is detected, p53 is activated, leading to cell cycle arrest at the G1 phase. This allows the cell to repair the damage or undergo programmed cell death (apoptosis) if the damage is irreparable. p53 exerts its effects by regulating the expression of target genes, including p21, which binds to and inhibits the activity of cyclin-dependent kinases (CDKs), halting cell cycle progression.

**Why Each Wrong Option is Incorrect**
**Option A:** ATM (Ataxia-Telangiectasia Mutated) is a protein kinase that plays a key role in the DNA damage response, but it is not the primary gene responsible for cell cycle arrest.
**Option B:** pRb (retinoblastoma protein) is a tumor suppressor that regulates cell cycle progression by binding to and inhibiting the E2F transcription factor, but it is not directly involved in the response to DNA damage.
**Option C:** Cyclin-dependent kinases (CDKs) are essential for cell cycle progression, but they are not the genes that cause cell cycle arrest in response to DNA damage.
**Option D:** p16INK4a is a tumor suppressor that inhibits CDK4 and CDK6, but it is not the primary gene responsible for cell cycle arrest in response to DNA damage.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that mutations in the TP53 gene, which encodes p53, are associated with a high risk of developing cancer, particularly in Li-Fraumeni syndrome. This highlights the critical role of p53 in maintaining genomic stability and preventing cancer.

**Correct Answer:** C. p53.