Speaking as a paramedic, I'll say this...
It's funny how often we get asked "what's the worst call you've ever been on?" which, when you think about it, is a strange question to ask. Do you go around to other people in your life on social occasions to ask them to relive unpleasant events for your curiosity?
However, that being said:
It's generally assumed that it's the most gruesome call. The most unpleasant trauma. That's not the case. When it comes down to it, we're all just tissue and blood. We stop bleeding, put (some) things back where they belong, and try to replace what has been lost (fluids), etc.
The worst calls are generally the tragic, the emotional abuse. Child sexual assault. Elder neglect. The horrible accidents that are no-one's fault, but will rack someone with guilt for the rest of their lives.
Those ones add up. I'd say those are the reasons for most EMS and Fire PTSD.
There's a good documentary about the rebirth of Detroit, which was for a long time the number one city for arson in the world, called "Burn" (incidentally financed/championed by Denis Leary)...
One of the firefighters in that documentary says, sitting, emotionally exhausted and eyes shallow from a long night, or career:
> I wish my head could forget what my eyes have seen.
https://youtu.be/9fCMogDYtus?si=F2kLr-aGb1f_W8Ds&t=23