Final answer:
An A-a gradient of 100 suggests a ventilation-perfusion mismatch, indicating an issue with gas exchange in the lungs, typically associated with conditions like pulmonary embolus, COPD, or pneumonia.
Step-by-step explanation:
If a patient breathing ambient air has an A-a gradient of 100, this finding is indicative of a ventilation-perfusion (V/Q) mismatch. The A-a gradient refers to the difference between the alveolar concentration of oxygen and the arterial concentration of oxygen. A normal A-a gradient is typically between 5-15 mmHg for a young adult breathing room air, but it can increase with age.
An elevated A-a gradient suggests that there is a problem with the exchange of gases in the lung's alveoli, often due to a mismatch between the areas of the lung receiving air (ventilation) and those receiving blood (perfusion). Conditions such as pulmonary embolus, chronic obstructive pulmonary disease (COPD), or pneumonia can lead to a V/Q mismatch and an increased A-a gradient.
Therefore, the correct answer to the question is d) A ventilation-perfusion mismatch.