For me, it's injections, hypodermic needles. Not pricking my finger by accident, or anything like that, but for vaccinations or blood tests, I will lock my muscles, shut right up, and make a spirited attempt to crush the arm rest of the chair until it's over. According to my mum, when I was younger, they tried to get a blood sample from my heel, but they kept pricking because they couldn't find anything, and apparently I screamed the ward down. That's probably the cause of it.