What chemical/element, if it was injected into the body, would actually burn you inside to the point of death?

I am writing a book, and in a part of it, a man is injected with a chemical that literally burns him up inside until he dies. Originally I have SULFURIC ACID as the substance injected into him, but now I'm not sure if that would happen with sulfur. I was looking at different elements like potium, lithium, the reactive elements, but what element/chemical would work best as an internal burner, per say??