Is it possible that a diagnosis of MG could be wrong and the real problem is MS? My husband has developed more symptoms like extreme weakness, inability to stand up, tremors and dry mouth.

Yes, it is possible. In fact, a person can have both MG and MS.

If your husband has had an MRI, the doctor should have been able to tell if it is MS. Not always though. Sometimes MRIs get misinterpreted, or the signs of MS don't show up.

Dry mouth isn't a typical symptom of MS. It's often related to a medicine the person is taking. It can be caused by mouth-breathing too.

I have dry mouth but it can be managed with one of the oral moisturizers that are on the market.

I hope your husband will be able to find out the cause of his symptoms soon.

How old is your husband? I just read that MG is more common in men older than 60.

To discard MS you'll need a lumbar puncture and a MRI (with contrast) of the brain and the spinal cord.