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
Gas used to measure the diffusion capacity of lung 
Select and explain. 
A. CO, B. NO, C. CO2, D. Nitrogen
A: CO
Answer: A: CO 
Explanation: Diffusion Capacity. Diffusion capacity assesses how well a tracer gas in inspired air can cross from the air into the blood. The test measures the absorption of a low concentration of carbon monoxide in inhaled air by hemoglobin in red blood cells that circulate through pulmonary capillaries. Ref: guyton and hall textbook of medical physiology 12 edition page number: 348,349,350