[Myrddin]

A vesicant is a drug or other agent that produces blisters.

Vesicants include mustard gas, phosgene oxime and lewisite. Comparing the available symptoms listed from the reporter that callapsed, lewisite was the best match. The itching, blanching and collapse (possibly from hypotension) were the key discriminators.

[Lucy Lake]

CDC Facts About Lewisite

http://www.bt.cdc.gov/agent/lewisite/basics/facts.asp

http://www.bt.cdc.gov/agent/lewisite/index.asp

Even more on google.