Final answer:
Claviceps purpurea is the fungus responsible for producing the toxin associated with St. Anthony's Fire (ergotism), affecting grain like rye and causing severe health issues.
Step-by-step explanation:
The organism that produces the toxin associated with St. Anthony's Fire (ergotism) is Claviceps purpurea. This fungus grows on rye and related grains, and it produces a mycotoxin known as ergot toxin. Ergotism comes in two forms, gangrenous and convulsive. The gangrenous form causes vasoconstriction which can lead to gangrene, whereas the convulsive form affects the central nervous system resulting in symptoms such as mania and hallucinations. The other options listed: Aspergillus nidulans, Amanita phalloides, and Stachybotrys chartarum are not associated with ergotism.