**Core Concept**
Retinoblastoma is a malignant tumor of the retina, and patients with this condition are at an increased risk of developing secondary malignancies due to genetic mutations. The second most common malignancy in patients with retinoblastoma is related to the same genetic predisposition.

**Why the Correct Answer is Right**
The correct answer is osteosarcoma, which is a type of bone cancer. This is because retinoblastoma is often caused by mutations in the RB1 gene, which is a tumor suppressor gene. Individuals with retinoblastoma are at a higher risk of developing osteosarcoma due to the loss of function of the RB1 gene, leading to uncontrolled cell growth and tumor formation. Osteosarcoma is a common secondary malignancy in patients with retinoblastoma, particularly in those with a bilateral or familial form of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Osteosarcoma is the correct answer, but other types of bone cancer, such as Ewing's sarcoma, are less common secondary malignancies in patients with retinoblastoma.
**Option B:** Leukemia is a type of blood cancer that is not typically associated with retinoblastoma.
**Option C:** Soft tissue sarcomas are a group of cancers that arise from connective tissue, but they are not the second most common malignancy in patients with retinoblastoma.
**Option D:** Brain tumors, such as gliomas, are not typically associated with retinoblastoma.

**Clinical Pearl / High-Yield Fact**
Patients with retinoblastoma should be closely monitored for signs of secondary malignancies, particularly osteosarcoma, and should undergo regular screening and follow-up to detect any potential tumors early.

**Correct Answer: C. Soft tissue sarcomas are a group of cancers that arise from connective tissue, but they are not the second most common malignancy in patients with retinoblastoma.**