**Core Concept**
Retinoblastoma is a malignant tumor of the retina, typically affecting children. The development of a second malignancy in retinoblastoma survivors is a significant concern due to the genetic predisposition associated with this disease.

**Why the Correct Answer is Right**
The most common second malignancy in survivors of retinoblastoma is osteosarcoma. This is because retinoblastoma is associated with a mutation in the RB1 tumor suppressor gene, which is also involved in the regulation of cell growth and division in other tissues, including bone. Survivors of retinoblastoma are at an increased risk of developing osteosarcoma, particularly during their second to fourth decades of life. This is thought to be due to the accumulation of additional genetic mutations in the remaining RB1 gene, leading to uncontrolled cell growth and tumor formation.

**Why Each Wrong Option is Incorrect**
**Option A:** Melanoma is a possible second malignancy in retinoblastoma survivors, but it is less common than osteosarcoma. Melanoma arises from the skin, and while it is a concern in retinoblastoma survivors, it is not the most common second malignancy.
**Option B:** Breast cancer is a common malignancy in the general population, but it is not typically associated with retinoblastoma. While retinoblastoma survivors may be at an increased risk of certain cancers, breast cancer is not the most common second malignancy in this group.
**Option C:** Soft tissue sarcomas are a group of rare cancers that arise from soft tissues such as muscle, fat, and blood vessels. While retinoblastoma survivors may be at an increased risk of certain soft tissue sarcomas, they are not the most common second malignancy in this group.

**Clinical Pearl / High-Yield Fact**
Retinoblastoma survivors should be monitored closely for signs of osteosarcoma, including bone pain, swelling, or deformity. Regular imaging studies and bone scans may be necessary to detect osteosarcoma at an early stage, when it is more treatable.

**Correct Answer:** C. Osteosarcoma.