**Core Concept**
Retinoblastoma is a malignant tumor of the retina, primarily affecting children. The risk of developing a second malignant neoplasm (SMN) is significantly higher in survivors of retinoblastoma due to genetic predisposition, particularly in those with germline mutations. The second malignancies often arise in organs with high cell turnover.

**Why the Correct Answer is Right**
The most common second malignant neoplasm in survivors of retinoblastoma is osteosarcoma. This is attributed to the inactivation of the RB1 tumor suppressor gene, which is responsible for regulating cell growth and division. When the RB1 gene is mutated, cells may become uncontrolled, leading to the development of osteosarcoma. Osteosarcoma is a type of bone cancer that often presents as a painful, rapidly growing tumor in the long bones of the extremities.

**Why Each Wrong Option is Incorrect**
**Option A:** Leukemia is a common second malignancy in survivors of retinoblastoma, but it is not the most common.
**Option B:** Soft tissue sarcomas can also occur as second malignancies, but they are less common than osteosarcoma.
**Option C:** Brain tumors are a potential risk in survivors of retinoblastoma, but they are not the most common second malignant neoplasm.

**Clinical Pearl / High-Yield Fact**
Retinoblastoma survivors are at increased risk of developing osteosarcoma due to genetic predisposition and the inactivation of the RB1 tumor suppressor gene. This highlights the importance of lifelong surveillance and monitoring for secondary malignancies in these patients.

**Correct Answer:** C. Osteosarcoma.