**Core Concept**
The diagnosis of Cytomegalovirus (CMV) retinitis primarily involves identifying the characteristic histopathological features and clinical manifestations of the disease. CMV retinitis is a serious opportunistic infection that affects immunocompromised individuals, particularly those with advanced HIV/AIDS.

**Why the Correct Answer is Right**
The correct answer is **Fundoscopic examination**, as it allows for the visualization of characteristic lesions such as retinal necrosis, hemorrhage, and perivasculitis. Fundoscopic examination is a crucial diagnostic tool for CMV retinitis, enabling clinicians to identify the characteristic "pizza pie" or "owl's eye" lesions. These lesions are caused by the viral replication and subsequent inflammatory response in the retinal tissue.

**Why Each Wrong Option is Incorrect**
**Option A:** **Fluorescein angiography** is a useful diagnostic tool for various retinal disorders, but it is not the primary method for diagnosing CMV retinitis.
**Option B:** **Electroretinography** is a test used to measure the electrical activity of the retina, but it is not specific for diagnosing CMV retinitis.
**Option C:** **Histopathological examination** is essential for confirming the diagnosis of CMV retinitis, but it is not the primary method used for initial diagnosis.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that CMV retinitis typically affects immunocompromised individuals, particularly those with advanced HIV/AIDS, and is often associated with a CD4 count below 100 cells/μL.

**Correct Answer:** C. Fundoscopic examination.