This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory System, Subject name: Physiology
A 54-year-old man sustains third degree burns in a house fire. His respiratory rate is 30/min, Hb = 17 g/dL, arterial PO2 is 95 mm Hg, and arterial O2 saturation is 50%. The most likely cause of his low oxygen saturation is: 
Select and explain. 
A. Airway obstruction from smoke inhalation, B. Carbon monoxide poisoning, C. Pulmonary edema, D. Fever
B: Carbon monoxide poisoning
Answer: B: Carbon monoxide poisoning 
Explanation: Ans. B. Carbon monoxide poisoningCarbon monoxide will lower oxygen content and saturation, but the arterial PO2 is unchanged. Airway obstruction or pulmonary edema will lower O2 saturation and arterial PO2 .