As-73 if

Arsenic nuclei have an odd number of protons in them. They also have a lot more protons in them than iron nuclei, which are the heaviest nuclei made in any sort of number by stars.

There is no way that organisms would preferentially incorporate arsenic rather than phosphorus unless they were on a planet where somehow all of the phosphorus had been sequestrated by some unfathomable geological process or naughty alien scientists.

That is all I have to say.