After stroke, MRI shows surprising extensive white matter damage

Hello,
My mom recently suffered a stroke and underwent some tests to find out the cause. It turns out the stroke was not the main problem. The doctors determined from her MRI scan that she has extensive white matter damage to her brain, which is surprising because she doesn't outwardly display any affects of the disease; she walks fine, has no trouble making decisions, etc. The neurologist said there were three possible diagnoses- multiple sclerosis, vasculitis, or another autoimmune disease. My mom is fairly young to have suffered a stroke (she is under 50) and does not recall experiencing any symptom that would indicate damage to her brain. What other diseases could be the cause of this and what tests are typically done to reach a diagnosis? We are afraid that it might be Multiple Sclerosis. Just posting this to reach out to any neurologist or doctor who has any opinion to offer on the situation or any others that have or are experiencing anything similar
thank you