Abdominal ultrasound. If a CAT scan and an MRI hasn't told me what the heck is going on, I don't have much hope that an ultrasound will either. But then I have had my hopes up too many times only to realize that nope, that's not it either.
Frankly, I have been surprised how being ill and trying to find out what is causing it just takes over one's life. For one thing it is time consuming to go to multiple doctor's offices, find a parking place & go in, wait in the interminable waiting rooms and exam rooms, then drive home again or back to work. Then there is the time spent on the phone trying to get something scheduled or talk to the doctor's nurse or waiting for a call back.
And finally, there is the emotional toll. I have never been so depressed in all my life. I think I could deal with most anything if I just knew what it was.
I am extremely fortunate that I have excellent health insurance and access to medical care. I flat don't know how on earth people deal with serious illness and a financial nightmare of poor or no insurance at the same time.