This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Toxicology, Subject name: Forensic Medicine
Amyl nitrate is used as an antidote in which poisoning? 
Select and explain. 
A. Arsenic poisoning, B. Hydrocyanide poisoning, C. Lead poisoning, D. Yellow poisoning
B: Hydrocyanide poisoning
Answer: B: Hydrocyanide poisoning 
Explanation: Ans. is 'b' i.e., Hydrocyanide poisoning * Amyl nitrite is used as an antidote in poisoning of hydrocyanic acid (cyanide).* Nitrates generate methaemoglobin which has high affinity for cyanide and forms cyanmethaemoglobin.* Cyanmethaemoglobin may again dissociate to release cyanide, therefore, sodium thiosulfate is given to form sod. thiocyanate which is poorly dissociable and is excreted in urine.