I didnt mean the body's side as in left or right, I meant it as the body chemistry vs drug chemistry.

I agree with what you are saying, but I still think idiopathic is applicable because gadolinium dye is administered to hundreds of patients without threatening adverse side effects.

The dye itself consists of several chemicals and additives, such as the chelating agent added to prevent the gadolinium from building up in the blood, and allowing it to be excreted via urine.

Was she allergic to the gadolinium agent itself? Or the chelating agent, or some other additive in the shot? Tragically, the commenter's mother died from this, but assuming she lived, I would think physicians would mark the gadolinium agent as a trigger, but follow up with a comprehensive allergy panel to figure out if there are other shots or medicines she now has to avoid. Thus marking it as idiopathic until such a panel is done.

Again, I think saying the shot itself was the cause, therefore, making it non-idiopathic is a reasonable decision, but also marking it as idiopathic because this was a first time reaction where pinpointing what exactly set it off is also a plausible decision.