Final answer:
The toxic pufferfish served in Japan, which requires a chef to have a license, is known by the four-letter name fugu. It contains a potent neurotoxin called tetrodotoxin.
Step-by-step explanation:
In Japan, a chef must pass a national exam for a license to serve the toxic pufferfish, which is also known by the four-letter name fugu. This is a precautionary measure due to the highly poisonous nature of certain parts of the fish, which contain tetrodotoxin, a potent neurotoxin. Chefs must be highly trained to prepare fugu safely, as improper preparation can lead to fatal consequences.