There are more than 100 diseases that can mimic MS including the lesions in the brain, so its a process, and not normally a big jump. Most MDs will start ruling out the stuff they can rule out, like vitamin def stuff. B12/D and so on. Then they look for inflammation markers for RA, sjoghrens, lupus, lyme...if those bear out negative they move onto the MRI studies and look to see how many lesions, and where they are placed in your brain or spinal canal.

Most will take a spinal tap as well. (normally only positive if taken while you are IN a flare of sx) They follow your MRI from today to the next three or six months and look for changes. There are some MDs that will dx you simply based on your symptoms only, and wont keep you on limbo isle, but its more common for them to be completely sure. Once they write MS on your medical form its life changing. You cant get insurance or life insurance on your own. your employement is at risk in some cases, and it can be a blessing or a curse. A blessing to help you get help, but a curse if you want insurance.

I wish you luck. its a long road, but many of us know each step and are happy to walk with you.