**Core Concept**
Neuromyelitis optica (NMO), also known as Devic's syndrome, is a rare autoimmune inflammatory demyelinating disease primarily affecting the optic nerves and spinal cord. It is characterized by recurrent episodes of optic neuritis and myelitis, often accompanied by significant disability.

**Why the Correct Answer is Right**
Devic's syndrome is a distinct clinical entity from multiple sclerosis (MS), although it can present with overlapping symptoms. The key distinguishing feature is the presence of aquaporin-4 (AQP4) antibodies, which are commonly found in NMO patients. These antibodies target the astrocytic water channels in the central nervous system (CNS), leading to inflammation and damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Optic neuritis is a hallmark feature of Devic's syndrome, often presenting as sudden, unilateral vision loss. This makes option A incorrect.
**Option B:** Myelitis, or inflammation of the spinal cord, is another characteristic feature of Devic's syndrome. This makes option B incorrect.
**Option C:** AQP4 antibodies are a key diagnostic marker for Devic's syndrome, making option C incorrect.

**Clinical Pearl / High-Yield Fact**
To differentiate Devic's syndrome from MS, consider the presence of AQP4 antibodies and the typical clinical presentation of optic neuritis and myelitis. In MS, optic neuritis is typically bilateral and myelitis is less common.

**Correct Answer: D.**