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
Aerial blood gas of a 5-year-old child done at sees level gives the following results: pH 7.41, PaO2 100 mgHg, and PaCO2 40 mm Hg. The child is being ventilated with 80% oxygen. What is the (A-a) PO2? 
Select and explain. 
A. 570.4 mm Hg, B. 520.4 mm Hg, C. 470.4 mm Hg, D. 420 mm Hg
D: 420 mm Hg
Answer: D: 420 mm Hg 
Explanation: The alveolar gas equation is a formula used to approximate the paial pressure of oxygen in the alveolus (PAO2):PAO2=(PB-PH2O)FiO2-(PaCO2/R)where PB is the barometric pressure, PH2O is the water vapor pressure (usually 47mmHg), FiO2 is the fractional concentration of inspired oxygen, and R is the gas exchange ratio. Ref: guyton and hall textbook of medical physiology 12 edition page number:331,332,333