**Core Concept**
Pthisis bulbi, also known as phthisis bulbi, is a condition characterized by the shrinkage and atrophy of the eyeball, often resulting from severe ocular trauma, intraocular inflammation, or retinal detachment. This condition can lead to a range of complications, including vision loss and pain.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of pthisis bulbi. The eyeball undergoes shrinkage due to the degeneration of the ocular tissues, including the retina, choroid, and sclera. This process can be triggered by severe ocular trauma, intraocular inflammation, or retinal detachment, leading to a loss of vision and potential pain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is a common symptom of pthisis bulbi, where the eyeball becomes severely inflamed and painful due to the degeneration of the ocular tissues.
**Option B:** This option is a potential cause of pthisis bulbi, where severe ocular trauma can lead to the shrinkage and atrophy of the eyeball.
**Option D:** This option is a characteristic feature of pthisis bulbi, where the eyeball undergoes shrinkage and atrophy due to the degeneration of the ocular tissues.

**Clinical Pearl / High-Yield Fact**
It's essential to note that pthisis bulbi can be a complication of untreated or severe ocular trauma, emphasizing the importance of prompt medical attention in such cases.

**Correct Answer: C.**