**Core Concept**
Sympathetic ophthalmitis is a rare, bilateral granulomatous inflammation that occurs following penetrating eye injury or surgery, leading to inflammation in the uninjured eye (sympathizing eye). This condition involves an immune response, where the body's immune cells attack the healthy eye's tissues, believing them to be foreign.

**Why the Correct Answer is Right**
The earliest symptom of sympathetic ophthalmitis is often a decrease in vision in the uninjured eye. This decrease in vision is caused by the immune-mediated inflammation in the sympathetic eye. The inflammation leads to damage to the retina and optic nerve, resulting in vision loss. The pathophysiology involves the activation of T lymphocytes, which recognize the ocular antigens as foreign and initiate an immune response.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is not the earliest symptom of sympathetic ophthalmitis. While pain can occur in the affected eye, it is not typically the first symptom noticed.

**Option B:**
This option is also not the earliest symptom. Photophobia can occur as the inflammation worsens, but it is not the initial symptom.

**Option C:**
This option is incorrect. While vision loss can occur in sympathetic ophthalmitis, it is not the earliest symptom. The decrease in vision is often a result of the inflammation and damage to the retina and optic nerve.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that sympathetic ophthalmitis can occur even if the initial eye injury or surgery appears minor. This condition highlights the importance of monitoring patients closely after eye trauma or surgery, as early detection and treatment can significantly impact visual outcomes.

**Correct Answer: D. Decrease in vision.**