Well, I guess the earliest physical exams I can remember were when it first happened, at school exams where I undressed in front of the doctor. An assistant also came in and out, or the school nurse. I didn't mind them, but I remember feeling nervous, every time the door opened, that there were classmates right outside the door.
As an older student, I was more apprehensive as a nurse or someone would come in, while I was undressing or being examined naked, shorts down or gown off. But, afterwards, the memory would make me feel excited, and began to hope it would happen at my next exam.
As an adult, I've tried to make it happen, sometimes. Getting undressed more than I've been asked to, or not wearing underwear to an exam with a doctor's office where I've been before. They leave to get me a gown, and I'm naked when they return. In warm weather, when I'm only wearing shorts and a shirt, pulling those off so quick that the nurse or assistant haven't even left the room, yet.
When I'm a subject for medical studies, there have been lots of times when I'm undressing or naked when someone comes in. Some of these are studies where I'm going to be naked for all or part of it, but my heart still jumps a bit when someone opens that door.