Arsenic in the blood and urine...

I was talking with my mother earlier this morning, and she mentioned a patient she has (she’s a nurse in a nursing home). Apparently, the gentleman has recently been discovered to have arsenic in his blood and urine. As a result, of course, restrictions have been placed on his family’s abilities to bring food and/or beverages to him. The nursing staff now is solely responsible for bringing the patient anything to eat or drink. Apparently, according to my mother, someone (and probably family) was trying to kill the man.

The problem I have with this, however (and I’ll admit that I’ve watched too many detective shows), is this: If someone were to introduce arsenic into someone else’s food in order to kill him, wouldn’t they have tried a liberal dose at the onset, rather than gradually exposing him to it?

Put another way: Is there a way, short of someone trying to kill this man, that could account for arsenic in the bloodstream and urine? I don’t presume that it would occur naturally. But is it conceivable that it could be the result of medications or such?

Does arsenic in the blood and urine automatically indicate the presumption of foul play?