**Core Concept**
The question requires knowledge of systemic diseases that affect the retina. The retina is a complex neural tissue that can be affected by various systemic conditions, including infections, autoimmune diseases, and metabolic disorders. The correct answer will be a disease or condition that does not affect the retina.

**Why the Correct Answer is Right**
The correct answer is **C. Phaeochromocytoma**. Phaeochromocytoma is a rare tumor of the adrenal gland that secretes excess catecholamines, leading to hypertension and other systemic symptoms. However, it does not typically affect the retina. In contrast, the other options are all diseases that can affect the retina. For example, **A. Syphilis** can cause a range of ocular manifestations, including retinitis. **B. Sarcoidosis** is a systemic autoimmune disease that can affect the eyes, including the retina, through granulomatous inflammation. **D. Diabetes mellitus** is a metabolic disorder that can cause diabetic retinopathy, a leading cause of blindness worldwide.

**Why Each Wrong Option is Incorrect**
**Option A:** Syphilis is a systemic infection that can cause a range of ocular manifestations, including retinitis.
**Option B:** Sarcoidosis is a systemic autoimmune disease that can affect the eyes, including the retina, through granulomatous inflammation.
**Option D:** Diabetes mellitus is a metabolic disorder that can cause diabetic retinopathy, a leading cause of blindness worldwide.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that many systemic diseases can have ocular manifestations, and a thorough eye examination is crucial in the diagnosis and management of these conditions.

**Correct Answer:** C. Phaeochromocytoma