**Core Concept**
Multiple sclerosis (MS) is a chronic autoimmune disease characterized by demyelination, inflammation, and neurodegeneration in the central nervous system (CNS). The disease involves the immune system attacking the protective covering of nerve fibers, leading to impaired communication between the brain and the rest of the body.

**Why the Correct Answer is Right**
The earliest feature of MS is often the appearance of lesions in the CNS on magnetic resonance imaging (MRI). These lesions are typically periventricular, juxtacortical, or infratentorial, and may be associated with optic neuritis, which is an inflammation of the optic nerve. The lesions are a result of demyelination and inflammation caused by the immune system attacking the myelin sheath of nerve fibers. This process is mediated by autoreactive T-cells and macrophages, which recognize and attack the myelin basic protein (MBP) and other myelin components.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular feature of MS. While optic neuritis is a common symptom of MS, it is not the earliest feature.
**Option B:** This option is incorrect because it refers to a treatment for MS, rather than an early feature of the disease.
**Option C:** This option is incorrect because it refers to a complication of MS, rather than an early feature.

**Clinical Pearl / High-Yield Fact**
The McDonald criteria are widely used to diagnose MS, and involve the presence of at least one clinical event (symptom) and at least two out of four MRI features, including lesions in the periventricular area, juxtacortical lesions, infratentorial lesions, or at least one optic nerve lesion.

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