**Core Concept**
Retinoblastoma is a malignant tumor of the retina that can be either heritable or sporadic. The heritable form is caused by a germline mutation in the RB1 gene, while the sporadic form is caused by a somatic mutation. Determining the origin of the tumor is crucial for genetic counseling and risk assessment for the patient's family members.

**Why the Correct Answer is Right**
To determine whether the retinoblastoma is heritable or sporadic, a genetic test called **PCR (Polymerase Chain Reaction) for RB1 gene mutation** can be performed on a blood sample from the patient. This test can detect the presence of a germline mutation in the RB1 gene, which would indicate a heritable form of retinoblastoma. If the mutation is found, it means that the patient has a germline mutation and there is a higher risk of the condition being passed on to their children.

**Why Each Wrong Option is Incorrect**
**Option A:** Fluorescein angiography is an imaging test used to diagnose retinoblastoma by highlighting the tumor and visualizing its blood supply. However, it is not a genetic test and cannot determine whether the tumor is heritable or sporadic.

**Option B:** CT scan and MRI are imaging tests used to diagnose and stage retinoblastoma, but they do not provide information about the genetic origin of the tumor.

**Option C:** Ophthalmoscopy is a diagnostic tool used to visualize the retina, including tumors. It is not a genetic test and cannot determine whether the retinoblastoma is heritable or sporadic.

**Option D:** Ultrasound is a diagnostic tool used to visualize the eye and diagnose retinoblastoma. However, it is not a genetic test and cannot determine whether the tumor is heritable or sporadic.

**Clinical Pearl / High-Yield Fact**
It is essential to note that retinoblastoma can be caused by a mutation in the RB1 gene, which is a tumor suppressor gene. This mutation can be inherited in an autosomal dominant pattern, meaning that a single copy of the mutated gene is enough to increase the risk of developing retinoblastoma.

**Correct Answer:**. Genetic testing for RB1 gene mutation (PCR).