<s>[INST] context: Heating devices that burn hydrocarbons, such as petroleum products, generate carbon monoxide, which can build up to dangerous levels in unventilated or poorly ventilated houses. Chronic carbon monoxide poisoning produces central nervous system damage. Carbon monoxide binds much more tightly to the hemoglobin than does oxygen, resulting in hypoxia. Decreased mental functioning generally begins at carboxyhemoglobin levels greater than 20%, and death is likely at levels greater than 60%. The classic "cherry red" lividity is rare and more typical for severe acute poisoning. Acute exposure to beryllium may cause pneumonitis; chronic exposure results in a sarcoid like a pulmonary disease. Nitrous oxide compounds and ozone are found in smog and may cause respiratory discomfort with diminished respiratory function in very young individuals, very old individuals, and individuals with underlying respiratory diseases; but such exposure is not immediately life-threatening. In very high concentrations, oxygen can be toxic to the lungs and can promote diffuse alveolar damage. Sulfur dioxide emissions are a component of smog and promote acid rain; they may increase the risk of chronic bronchitis. [/INST] question: A 75-year-old man who lives alone in a poorly ventilated house without central heating uses a portable unvented kerosene heater to warm the house during the winter months. One morning, a neighbor finds him in an obtunded state. On physical examination, he appears cyanotic. Results of blood gas measurement on room air are PO2, 90 mm Hg; PCO2, 35 mm Hg; and pH, 7.3. Pulse oximetry shows low oxygen saturation. Exposure to which of the following is most likely to have produced this man's illness? | options: A: Beryllium B: Carbon monoxide C: Nitrous oxide compounds D: Oxygen | answer: B </s>