This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Forensic Medicine
Soot in trachea is a sign of? 
Select and explain. 
A. Postmortem burns, B. Cyanide poisoning, C. Antemortem burns, D. CO poisoning
B: Cyanide poisoning
Answer: B: Cyanide poisoning 
Explanation: Soot in external respiratory passages could be due to antemortem or postmortem burn. But if soot is seen deep in trachea, it is a sign of antemortem burn.