**Core Concept**
Ocular lymphoma is a type of cancer that arises from the lymphoid cells in the eye, often presenting with symptoms such as redness, vision loss, and painless proptosis. The condition is typically associated with systemic lymphoma or autoimmune disorders.

**Why the Correct Answer is Right**
Ocular lymphoma can manifest in various forms, including primary vitreoretinal lymphoma, which often presents with painless, unilateral, or bilateral ocular symptoms such as vision loss, floaters, or light perception. In contrast, conjunctival lymphoma typically presents with painless, unilateral redness, and can be associated with systemic symptoms. However, one symptom that is less commonly associated with ocular lymphoma is **severe eye pain**, which is more often seen in other conditions such as acute angle-closure glaucoma or endophthalmitis.

**Why Each Wrong Option is Incorrect**
**Option A:** Vision loss is a common symptom of ocular lymphoma, particularly in primary vitreoretinal lymphoma.
**Option B:** Painless proptosis can be seen in ocular lymphoma, especially in cases of orbital lymphoma.
**Option C:** Redness and discoloration are common presenting symptoms of ocular lymphoma, as seen in the patient's case.

**Clinical Pearl / High-Yield Fact**
It's essential to note that ocular lymphoma can be challenging to diagnose, and a high index of suspicion is required, especially in patients with a history of systemic lymphoma or autoimmune disorders. A thorough ophthalmological examination, imaging studies, and biopsy are often necessary for a definitive diagnosis.

**Correct Answer:** D. Severe eye pain.