**Core Concept**
Optic neuritis is an inflammatory condition affecting the optic nerve, leading to vision loss and pain on eye movement. It is often associated with demyelinating diseases like multiple sclerosis.

**Why the Correct Answer is Right**
The best investigation for optic neuritis is the visual evoked potential (VEP) test, which measures the electrical signals generated by the brain in response to visual stimuli. This test helps to assess the function of the optic nerve and diagnose optic neuritis. The VEP test is particularly useful in cases where the visual acuity is severely impaired, making it difficult to perform other visual tests.

**Why Each Wrong Option is Incorrect**
**Option A:** MRI of the brain with contrast is not the best initial investigation for optic neuritis, although it may be performed later to rule out other conditions or complications.

**Option B:** Visual field testing is an important investigation in optic neuritis, but it is not the best initial test. It is used to assess the peripheral vision and detect any defects.

**Option C:** OCT (Optical Coherence Tomography) is a useful investigation in optic neuritis, but it is not the best initial test. It helps to assess the retinal nerve fiber layer and detect any abnormalities.

**Clinical Pearl / High-Yield Fact**
The visual evoked potential (VEP) test is a high-yield investigation for optic neuritis, and it is essential to remember that it is the best initial test to diagnose and manage this condition.

**Correct Answer:** C. OCT (Optical Coherence Tomography)