**Core Concept**
Multiple sclerosis (MS) is a chronic autoimmune disease characterized by demyelination, axonal loss, and progressive neurological deficits. The disease manifests with various symptoms, but the initial presentation often sets the stage for the subsequent clinical course.

**Why the Correct Answer is Right**
Optic neuritis is the most common initial presenting feature of multiple sclerosis, affecting approximately 50-70% of patients. It typically presents as sudden, unilateral vision loss, often accompanied by pain on eye movement. This symptom arises due to inflammation and demyelination of the optic nerve, which disrupts the visual pathway. The exact mechanism involves the breakdown of the blood-brain barrier, allowing autoreactive T cells and antibodies to target myelin basic protein (MBP) and other myelin components.

**Why Each Wrong Option is Incorrect**
**Option A:** Cognitive impairment is a common feature of MS, but it is not the most common initial presenting symptom. Cognitive dysfunction often emerges later in the disease course.
**Option B:** Sensory symptoms, such as numbness or tingling, can occur in MS, but they are not as common as optic neuritis as an initial presenting feature.
**Option C:** Ataxia or gait disturbances can be seen in MS, particularly in patients with a spinal cord lesion, but they are not the most frequent initial symptom.

**Clinical Pearl / High-Yield Fact**
The McDonald criteria, updated in 2017, provide a set of diagnostic criteria for multiple sclerosis, emphasizing the importance of clinical presentation, MRI findings, and the presence of oligoclonal bands in cerebrospinal fluid.

**Correct Answer: C. Optic neuritis is the most common initial presenting feature of multiple sclerosis.**