**Core Concept**
Devic's disease, now known as neuromyelitis optica (NMO), is an autoimmune disorder primarily targeting the central nervous system, especially the optic nerves and spinal cord. It is characterized by the presence of anti-aquaporin-4 (AQP4) antibodies, which attack AQP4 glycoprotein channels in astrocytes.

**Why the Correct Answer is Right**
Aquaporin-4 (AQP4) is the primary water channel expressed in astrocyte end-feet surrounding blood vessels in the central nervous system. In NMO, autoantibodies against AQP4 bind to these channels, triggering complement activation, astrocyte damage, and inflammation, leading to demyelination and severe neurological deficits. This antibody-mediated attack is the hallmark of the disease and is used diagnostically.

**Why Each Wrong Option is Incorrect**
Option A: Aquaporin 0 is expressed in the kidney and testes, not in the CNS, and is not involved in NMO.
Option B: Aquaporin 1 is found in renal collecting ducts and red blood cells, with no role in central nervous system immunity.
Option C: Aquaporin 2 is located in the collecting ducts of the kidney and regulates water reabsorption; it is unrelated to CNS autoimmunity.

**Clinical Pearl / High-Yield Fact**
Always remember: NMO is distinguished from multiple sclerosis by anti-AQP4 antibodies. AQP4-IgG testing is a key diagnostic tool, and patients often present with optic neuritis and transverse myelitis — a classic triad of symptoms.

✓ Correct Answer: D. Aquaporin 4