For 10 points, name this element with atomic number 33, a notable poison.
Cacodylic acid contains a central atom of this element. One drug that contains this element has a central ring of this element with phenamine groups hanging off, while its trioxide is used alongside ATRA to treat some leukemias. Besides being the central atom of Fowler's solution, it was use to treat syphilis and trypanosomiasis as part of Salversan. When combined with chlorine, it forms the chemical weapon lewisite, while when combined with gallium it can be used as a semiconductor. For 10 points, name this element with atomic number 33, a notable poison.
Cacodylic acid contains a central atom of this element. One drug that contains this element has a central ring of this element with phenamine groups hanging off, while its trioxide is used alongside ATRA to treat some leukemias.
{arsenic} [or As]