A 33 year old man is diagnosed with multiple sclerosis

Correct Answer: D

Explanation:
CNS myelin is an elaborate extension of the oligodendrocyte cell membrane A single oligodendrocyte myelinates as many as 20 or 30 different CNS axonal segments, each over a length of 1 mm or less. Oligodendrocyte membrane extensions wrap around the axons in a concentric fashion to form the myelin sheath. Brain, optic nerves, and spinal cord from MS patients contain scattered areas of myelin loss ranging in size from 1 mm to several centimeters in diameter. Although plaques may occur in any myelinated area of the CNS, the most commonly affected regions include optic nerves, periventricular cerebral white matter, and cervical spinal cord. The earliest event in development of the MS lesion is breakdown of the blood-brain barrier, followed by perivenular mononuclear infiltrates, and quickly thereafter by circumscribed areas of myelin breakdown. Macrophages invariably occupy sites of active demyelination and appear necessary for myelin loss. B lymphocytes and plasma cells surround small CNS blood vessels, and T lymphocytes and monocytes infiltrate CNS parenchyma. Tissue edema reaches a maximum after about 1 month, after which lesions evolve over several months into permanently demyelinated gliotic scars depleted of oligodendrocytes. After the initial events, immature oligodendrocytes appear and presumably participate in remyelination.