Alex Trebek: This compound is highly toxic with an LD50 of about 10mg. It's featured in lots of books and movies as an extremely bitter colorless compound found in nature. Its mechanism of action is to act on a ligand-gated chloride channel. Thus, you might expect that soon after exposure, the muscles in the head and neck begin to contract involuntarily. Soon, the patient will look like this painting below (which depicts a similar disease). Death occurs in hours from asphyxiation, and there is no antidote.
Challenge: What is...? (the compound shown above)
First image shown under GNU Free Documentation License.
Second image is in the public domain. It is a painting by Sir Charles Bell of a disease with a similar clinical picture.