We used the following question in a large classroom session attended by undergraduate medical students and doctors with a Bachelor of Medicine and Bachelor of Surgery (MBBS) degree (240 in all) to test for conceptual understanding as to what constitutes arterial blood oxygenation. The question read as follows: Which one of the following physiological parameters taken alone tells you that arterial blood oxygenation in a critically ill patient is satisfactory? A. (Alveolar-arterial) O(2) gradient=10 mmHg. B. Partial pressure of O(2) in arterial blood=95 mmHg. C. O(2)saturation of hemoglobin>90%. D. Blood hemoglobin concentration=12 g/dl. Only 25 of 240 students correctly indicated that none of the above parameters taken alone could give us this information. Once students turned in their answers, we presented five examples illustrating how none of the above answers could be used alone to assess arterial blood oxygenation. Students were then asked to provide written feedback on their understanding of this topic. The majority of students indicated that they were satisfied that they got rid of a misconception.