My thoughts changed as I got older. At first probably between the ages of 5 and 8, there was overwhelming worry about receiving a shot so that I only focused on mom protecting me from that eventuality. Being undressed seemed to take second place to having a shot. Of course, I was still required to undress and often be bare during a rectal temp and shot.
Between 9 and 10, I was aware of having to strip and often protested to no avail. Mom then was the enforcer of the nurse or doc's request.
At 11 and 12, it was total humiliation. Somewhere in that age range came the realization of reacting to the exam with shock and embarrassment due to genital reactions. Having one's parent observe the onset of puberty is mighty upsetting.
From 13 on, it was total terror as erections were inevitable during the exam. Trying to shield my penis was of little value. It was not automatic for me to do so as the sense of being naked was more prevailing. It also took a while to recognize how having the thermometer nestled in my butt could trigger an erection. I am not sure which came first, the being naked or having the thermometer inserted and waiting.
Overall, the impact of being examined became imprinted as a result of having no real control over what was to be done. By the time, I realized the difference between being "Undressed," or having to remove some clothing, I was well aware of feeling embarrassed at being stripped in front of a parent, doctor or nurse. The experience of watching the nurse weigh me - be it bare or partially undressed - only mattered for a few minutes because there was more to follow that would inevitably require my butt to be fully exposed. Seeing her lube the thermometer did little to dissuade me from feeling okay. The visual confirmation of the thermometer or her holding a syringe just made me tense and shake. The moment that either one took place just reinforced the process of being submissive, which now has become a strong factor in arousal.
This is a wonderful topic and hope to read how others have come to process the exam and associated memories and anticipation of new exams now.
For me the arousal factor is reinforced through role play or imaginary rehearsal in the days leading up to an exam. It is as if the visual recollection of standing in front of the doc or nurse having my briefs lowered and/or removed is happening again.