**Core Concept**
The patient's presentation of painless sudden loss of vision suggests a possible vascular or optic nerve-related issue. This condition can be caused by an ischemic event or a tumor affecting the optic nerve.

**Why the Correct Answer is Right**
The most probable diagnosis in this case is an optic neuritis, which is an inflammation of the optic nerve. This condition can be caused by a demyelinating disease such as multiple sclerosis (MS), a viral infection, or an autoimmune condition. The pathophysiology involves damage to the myelin sheath surrounding the optic nerve, leading to impaired nerve conduction and vision loss. The lack of systemic and ocular examination findings makes MS a possible diagnosis, given its often unpredictable and asymmetric presentation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a posterior vitreous detachment (PVD) typically presents with flashes of light and floaters, not sudden painless vision loss.

**Option B:** This option is incorrect because a retinal detachment would likely present with symptoms such as flashes of light, floaters, and a curtain-like shadow in the visual field, not just sudden painless vision loss.

**Option C:** This option is incorrect because a cataract would typically present with gradual vision loss, especially in low light conditions, rather than sudden painless vision loss.

**Clinical Pearl / High-Yield Fact**
Optic neuritis is often associated with multiple sclerosis, a condition that can affect young adults and is characterized by unpredictable and asymmetric neurological symptoms.

**Correct Answer:** C. Optic neuritis.