Which statement best defines the A-a gradient and what an elevated A-a gradient implies?

Which statement best defines the A-a gradient and what an elevated A-a gradient implies?

• A. A-a gradient = PAO2 − PaO2; PAO2 = FiO2 × (PB − PH2O) − PaCO2/R; an elevated A-a gradient indicates diffusion impairment, V/Q mismatch, shunt, or aging.
• B. A-a gradient = PaO2 − PaO2; PAO2 = FiO2 × (PB − PH2O) − PaCO2/R; an elevated gradient indicates improved oxygenation.
• C. A-a gradient = PAO2 − PaO2; PAO2 is simply FiO2 × PB; an elevated gradient means nothing abnormal.
• D. A-a gradient cannot be calculated in practice.

Answer: (A)