Three times, I have had a doctor pull down my underwear. The first time, I was very young and had chicken pox, and my male pediatrician pulled down my panties while I was standing to see if I had any bumps under there. The second time, I was just getting into puberty, around age twelve or so, and a female doctor I only saw once pulled my panties and shorts down together for just a second saying she needed to check to make sure I was developing okay.
The last time was in college, I'd gone to the campus clinic for abdominal pain. Most doctors there are very concerned about modesty and don't even ask you to change into a gown, but this one time I was in a gown and panties with a sheet draped over my hips. After examining my tummy, he pulled the sheet and my underwear down a few inches to palpate a little lower. It was a pleasant surprise!