**Core Concept**
The minimum alveolar concentration (MAC) is a measure of the potency of an inhaled anesthetic, representing the concentration of vapor in the lungs required to prevent movement in response to a painful stimulus in 50% of subjects. It is an important concept in anesthesiology, as it helps to determine the safe dosage of anesthetic gases.

**Why the Correct Answer is Right**
MAC is increased in conditions where there is a decrease in the number of functional anesthetic receptors in the brain, such as hypothermia. Hypothermia decreases the metabolism of the anesthetic, resulting in a higher MAC. This is because the brain is less responsive to the anesthetic at lower temperatures, requiring a higher concentration of the anesthetic to achieve the same effect.

**Why Each Wrong Option is Incorrect**
**Option A:** Obesity is not directly related to an increase in MAC. While obesity may affect the distribution and metabolism of anesthetics, it is not a primary factor in increasing MAC.
**Option B:** Respiratory acidosis may affect the binding of anesthetics to their receptors, but it is not a direct cause of an increase in MAC. In fact, some studies suggest that respiratory acidosis may decrease MAC.
**Option C:** Chronic obstructive pulmonary disease (COPD) may affect lung function and gas exchange, but it is not a direct cause of an increase in MAC.

**Clinical Pearl / High-Yield Fact**
Remember that MAC is affected by factors such as age, body temperature, and the presence of certain medical conditions. It is essential to consider these factors when determining the safe dosage of anesthetic gases.

**Correct Answer:** C. Hypothermia.