**Core Concept**
Multiple sclerosis (MS) is a chronic autoimmune disease characterized by demyelination of the central nervous system (CNS), leading to various neurological symptoms. The disease is marked by relapses or exacerbations, followed by periods of partial or complete recovery, known as remissions.

**Why the Correct Answer is Right**
The most common presentation of MS is optic neuritis, an inflammatory condition affecting the optic nerve. This results in acute vision loss, pain on eye movement, and sometimes even color vision impairment. The exact pathophysiology involves the breakdown of the blood-brain barrier, allowing autoreactive T cells to infiltrate the CNS and attack the myelin sheath surrounding nerve fibers. This disrupts normal nerve conduction, leading to various symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** While fatigue is a common symptom in MS, it is not the most common initial presentation. Fatigue can be a result of various factors, including decreased mobility, sleep disturbances, and anemia.

**Option B:** Sensory symptoms, such as numbness or tingling, can occur in MS, but they are not as frequent as optic neuritis as the initial presentation. Sensory symptoms are often a result of demyelination in the spinal cord.

**Option C:** Motor symptoms, like weakness or spasticity, can occur in MS, but they are not the most common initial presentation. Motor symptoms are often a result of demyelination in the spinal cord or brainstem.

**Option D:** Cognitive impairment can occur in MS, but it is not the most common initial presentation. Cognitive impairment is often a result of widespread demyelination in the brain.

**Clinical Pearl / High-Yield Fact**
The McDonald criteria, established in 2017, provide a framework for diagnosing MS, requiring evidence of dissemination in space (at least two lesions in different areas of the CNS) and dissemination in time (at least one episode of worsening neurologic function).

**Correct Answer:** C. Motor symptoms like weakness or spasticity can occur in MS, but they are not the most common initial presentation.