When I was 4 or 5 years old, I remember my doctor pulling my underwear down, briefly checking my penis and testicles (embarrassing but normal) then flipping me face down, all without any explanation. It was totally unfamiliar to me. Before that examination, I can remember the doctor just tugging the waistband of my underwear down to take a peek and make sure my testicles were descended.
But for this examination, my underwear was pulled down below my knees. Having my only thin layer of modesty dragged away like that was embarrassing, then being flipped onto my stomach was scary. Face down, I couldn't see what anybody was doing, and I didn't know what to expect. I waited a minute...nothing happening...then i feel the doctor open my bottom up and visually examine my anus. She used both hands and spread it wide. I felt a little bit of touching around the anus, which was kind of pleasant, then she let go and went across the room. "Just stay like that" she said, so I'm laying face down on the exam table with my bare butt exposed for everyone to see. Next I heard the plastic 'click' of a thermometer case. By that time I was usually getting my temperature taken under my arm, but this doctor took it rectally. She came over with the nurse and the nurse opened my bottom up and the doctor inserted the thermometer. It was a very long 3 minutes. The doctor held the thermometer in place, I could feel her hand resting on my bottom. Finally she took it out, wiped my bottom, told me I was a good boy and pulled my undies back up.
That exam is etched into my being, and is probably one of the biggest factors that gave me a medical fetish. I'd love to perform it. I remember it vividly. It wasn't particularly complex or painful, but it was embarrassing and completely unexpected. It excited me.