Hmm...blood and blood vessels. And this is an odd one, because the circumstances leading to a reaction vary quite a lot. I don't think I've ever had a reaction from a movie or a video game, even though I really don't like excessive gore. Also, I'm fine with cuts and nosebleeds too...However, hearing a nurse describe a person with blood gushing from their neck...almost fainted. Same thing in biology class, but now with a video of human embryos. Those black eyes and bulging veins...Third case I remember was also biology or health class, this time with me imagining the blood pumping through my veins...
It's quite rare for me to get these reactions, but there surely is a pattern here...Maybe it's the mental image of blood vessels that triggers a reaction, don't know...Still, I did try to induce the reaction once by thinking, but couldn't get such a strong one. Although, I was lying down, so that might've actually prevented it...