**Core Concept**
Diffuse retinoblastoma involving the entire globe in one eye and a small peripheral tumor in the other eye is a classic presentation of bilateral retinoblastoma. The management of such cases requires a multidisciplinary approach involving oncology, ophthalmology, and radiotherapy. The primary goal is to preserve vision in the better eye while controlling the disease.

**Why the Correct Answer is Right**
The ideal management of this patient involves **chemoreduction** followed by **focal therapy** to the smaller tumor in the left eye. Chemoreduction involves the administration of chemotherapy to reduce the size of the tumor, making it more amenable to focal therapy. Focal therapy can be achieved through laser photocoagulation, cryotherapy, or brachytherapy. This approach allows for the preservation of vision in the better eye while controlling the disease. The use of **vincristine, carboplatin, and etoposide** is a common chemoreduction regimen for retinoblastoma.

**Why Each Wrong Option is Incorrect**
* **Option A:** Enucleation (removal of the affected eye) is not the ideal management in this case as it would involve sacrificing the better eye.
* **Option B:** Radiation therapy alone may not be sufficient to control the disease in the better eye, and chemoreduction is often preferred to reduce the tumor size.
* **Option D:** Systemic chemotherapy alone may not be effective in controlling the disease in the better eye, and focal therapy is often required to manage the smaller tumor.

**Clinical Pearl / High-Yield Fact**
Retinoblastoma is the most common primary intraocular malignancy in children, and bilateral retinoblastoma is often associated with a germ-line mutation in the RB1 gene.

**Correct Answer:** C. Chemoreduction followed by focal therapy.