A 48-year-old man presents to the ED with a right
middle lobe pneumonia. His arterial blood gas on
room air taken on arrival is as follows:
• pH 7.55 (7.35–7.45)
• PCO2 18 mmol/L (35–45)
• PO2 70 mmol/L (80–100)
• HCO3
− 18 mmol/L (22–24)
Which ONE of the following is his A-a gradient?
A. 127 mmHg
B. 47 mmHg
C. 57 mmHg
D. 77 mmHg
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