Which chronic autoimmune disease attacks the myelin sheath in the CNS?

• A. Parkinson's disease
• B. Multiple sclerosis (MS)
• C. ALS
• D. Alzheimer's disease

Answer: (B)

This item tests understanding of autoimmune demyelination in the CNS. Multiple sclerosis is a chronic autoimmune disease in which the immune system targets the myelin sheath that surrounds axons in the brain and spinal cord. The attack damages myelin and the underlying oligodendrocytes, producing focal areas of demyelination called plaques. This disruption slows or blocks nerve conduction, leading to a wide array of neurological symptoms depending on where the lesions occur. Over time, remyelination can occur but may fail, and neurodegeneration can follow in progressive forms. In contrast, the other conditions involve different primary pathologies: Parkinson's disease centers on loss of dopaminergic neurons in the substantia nigra; ALS involves degeneration of motor neurons; Alzheimer's disease is characterized by amyloid plaques and tau tangles with cortical neuronal loss.