**Core Concept**
The arterial-alveolar oxygen gradient (A-a DO2) is a measure of the difference between the alveolar oxygen tension (PAO2) and the arterial oxygen tension (PaO2). It is used to assess the efficiency of gas exchange in the lungs. In this context, the A-a DO2 is being measured to evaluate the effectiveness of ventilation in a 5-year-old child.

**Why the Correct Answer is Right**
The alveolar oxygen tension (PAO2) can be calculated using the alveolar gas equation: PAO2 = (FiO2 x (PB - 47)) - (PaCO2 / 0.8), where FiO2 is the fraction of inspired oxygen, PB is the barometric pressure, and PaCO2 is the arterial carbon dioxide tension. In this case, FiO2 = 0.8 (since the child is being ventilated with 80% oxygen), PB = 760 mmHg (at sea level), and PaCO2 = 40 mmHg. Plugging these values into the equation, we get PAO2 = (0.8 x (760 - 47)) - (40 / 0.8) = 550 - 50 = 500 mmHg. The (A-a) DO2 is then calculated as the difference between PAO2 and PaO2, which is 500 - 100 = 400 mmHg.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.
* **Option D:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The A-a DO2 is an important parameter in the evaluation of patients with respiratory disease, especially those with conditions such as chronic obstructive pulmonary disease (COPD) or pneumonia. A high A-a DO2 suggests impaired gas exchange in the lungs, which can be due to various factors including ventilation-perfusion mismatch, diffusion defects, or alveolar-capillary membrane damage.

**Correct Answer: D. 400 mmHg**