Repeated USMLE Questions Step 1- 276

A 64-year-old man complains of severe headache, dizziness, nausea and deterioration of vision. Workup shows a decrease in oxygen carrying capacity with no change in PO2 of arterial blood.

Which of the following is most likely responsible for this patient’s symptoms?

A- Ozone

B- Carbon monoxide (CO)

C- Sulfur dioxide

D- Nitrogen


This is a typical presentation of carbon monoxide poisoning. It is characterized by decreased oxygen capacity with no change in PO2 of arterial blood. Elevated level of carboxyhemoglobin should confirm the diagnosis. CO poisoning could cause irreversible brain damage or even death if it happens during sleep. Carbon monoxide is a colorless, odorless and tasteless gas which could be accumulated in the body without noticing it until too late when symptoms occur.

Ozone toxicity leads to chest pain, cough, shortness of breath and sore throat. Sulfur dioxide toxicity is causing cough, runny nose and difficulty breathing due to swollen lungs. Nitrogen toxicity is very rare and is known as inert gas asphyxiation which is used as a suicide method.

The correct answer is B

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