**Core Concept**
Increased alveolar O2 concentration is a critical parameter in respiratory physiology, reflecting the balance between oxygen supply and demand in the lungs. In conditions where alveolar O2 concentration increases, it is essential to identify the underlying mechanism driving this change.

**Why the Correct Answer is Right**
The correct answer is related to conditions that lead to a decrease in the partial pressure of carbon dioxide (CO2) in the alveoli, allowing for an increase in the partial pressure of oxygen (O2). This occurs in conditions where there is a reduction in CO2 production or an increase in CO2 elimination. One such condition is **hyperventilation**, which is characterized by an increased rate and depth of breathing. This leads to a decrease in the partial pressure of CO2 in the blood, which in turn increases the alveolar O2 concentration. The mechanism behind hyperventilation involves the stimulation of the respiratory centers in the brain, leading to an increase in the respiratory rate and tidal volume.

**Why Each Wrong Option is Incorrect**
* **Option A:** While **anemia** can lead to an increase in the respiratory rate (polycythemia is a compensatory response to anemia), it does not directly cause an increase in alveolar O2 concentration.
* **Option B:** **Asthma** is a condition characterized by airway obstruction and inflammation, which can lead to a decrease in lung function and alveolar O2 concentration, not an increase.
* **Option D:** **Pneumonia** is an infection of the lungs that can lead to inflammation and consolidation of lung tissue, resulting in a decrease in alveolar O2 concentration.

**Clinical Pearl / High-Yield Fact**
Remember that hyperventilation can lead to respiratory alkalosis, a condition characterized by an increase in blood pH due to a decrease in CO2 levels. This can have significant clinical implications, including muscle weakness, tetany, and cardiac arrhythmias.

**Correct Answer: C. Hyperventilation**