Which antidote is used for cyanide poisoning?

The correct antidote for cyanide poisoning is sodium nitrate. Sodium nitrate plays an essential role in converting hemoglobin to methemoglobin, a form of hemoglobin that can bind cyanide more effectively than regular hemoglobin. This conversion helps facilitate the removal of cyanide from the body, as methemoglobin can then be metabolized and eliminated.

In the context of other options, calcium chloride is typically used to treat hypocalcemia or calcium channel blocker overdose, not cyanide poisoning. N-acetylcysteine is primarily used for acetaminophen overdose, providing hepatoprotection, and flumazenil is a benzodiazepine antagonist, used primarily in cases of benzodiazepine overdose. Each of these agents has specific applications but does not target cyanide poisoning effectively like sodium nitrate does.