What is the primary pathology of multiple sclerosis?

• A. Myelin sheath destruction leading to disrupted nerve impulse conduction.
• B. Degeneration of motor neurons in the spinal cord.
• C. Loss of dopamine-producing neurons in the substantia nigra.
• D. Autoimmune attack on acetylcholine receptors at the NMJ.

Answer: (A)

The main idea is that multiple sclerosis is an autoimmune attack that destroys the myelin sheath in the central nervous system. When myelin around axons is damaged, nerve impulses travel more slowly or can be interrupted, so signals between the brain and body become unreliable. This demyelination forms lesions in different CNS areas, so symptoms can vary widely depending on where the damage occurs. Over time, chronic demyelination can lead to axonal loss and lasting neurologic deficits, although some remyelination can occur, which may temporarily improve function. In contrast, the other conditions involve different primary processes—motor neuron degeneration in the spinal cord, loss of dopamine neurons in the substantia nigra, or autoimmune attack on acetylcholine receptors at the neuromuscular junction—so they don’t describe MS pathology.