**Core Concept**
Retinoblastoma is a malignant tumor of the retina, primarily affecting children. It is caused by mutations in a specific gene, leading to uncontrolled cell growth and tumor formation.

**Why the Correct Answer is Right**
The correct answer is RB1, the retinoblastoma gene. This tumor suppressor gene, located on chromosome 13q14, plays a crucial role in regulating the cell cycle. A mutation in the RB1 gene impairs its ability to inhibit cell proliferation, leading to the development of retinoblastoma. The RB1 protein interacts with E2F transcription factors to prevent excessive cell growth, and its inactivation allows for uncontrolled cell division.

**Why Each Wrong Option is Incorrect**
**Option A:** TP53 - While TP53 is a tumor suppressor gene involved in cell cycle regulation, mutations in TP53 are associated with Li-Fraumeni syndrome, which increases the risk of various cancers, but is not directly linked to retinoblastoma.

**Option B:** BRCA1/BRCA2 - These genes are primarily associated with breast and ovarian cancer susceptibility, not retinoblastoma.

**Option C:** PTEN - PTEN is a tumor suppressor gene involved in the regulation of the PI3K/AKT pathway, but its mutations are associated with Cowden syndrome and other disorders, not retinoblastoma.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that retinoblastoma can be inherited in an autosomal dominant pattern, meaning a single copy of the mutated gene is sufficient to increase the risk of developing the disease. This highlights the importance of family history and genetic counseling in the diagnosis and management of retinoblastoma.

**Correct Answer:** C. PTEN is not the correct answer.