**Core Concept**
Painless sudden onset of loss of vision is a clinical presentation that can be caused by various conditions affecting the eye or the brain. This presentation requires a thorough evaluation to determine the underlying cause.

**Why the Correct Answer is Right**
One of the common causes of painless sudden onset of loss of vision is **Optic Neuritis**. Optic neuritis is an inflammatory condition of the optic nerve, which can be caused by a viral or bacterial infection, multiple sclerosis, or other inflammatory conditions. The inflammation results in damage to the myelin sheath surrounding the optic nerve, leading to loss of vision. The painless nature of the vision loss in optic neuritis is due to the involvement of the central part of the optic nerve, which is not innervated by pain fibers.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a condition that typically presents with painless sudden onset of loss of vision. However, without more information, it's difficult to evaluate this option accurately.
* **Option B:** This option is incorrect because **Diabetic Retinopathy** typically presents with gradual vision loss, not sudden onset. While it can cause vision loss, it is not characterized by a painless sudden onset.
* **Option C:** This option is incorrect because **Central Retinal Vein Occlusion** typically presents with sudden painless vision loss in one eye, but it is often accompanied by other symptoms such as eye redness, swelling, and vision changes.

**Clinical Pearl / High-Yield Fact**
In cases of painless sudden onset of loss of vision, it's essential to perform a thorough ophthalmologic examination, including visual acuity tests, slit-lamp examination, and imaging studies such as MRI or CT scans to determine the underlying cause.

**Correct Answer:** B. Diabetic Retinopathy