**Core Concept**
Metabolic acidosis is a condition characterized by an excess of acid or a deficiency of base in the body, leading to a decrease in blood pH. In metabolic acidosis, the kidneys try to compensate by increasing the excretion of hydrogen ions, but this process also affects the regulation of carbon dioxide (CO2) levels.

**Why the Correct Answer is Right**
In metabolic acidosis, the body tries to compensate by increasing the excretion of hydrogen ions. This process is facilitated by the kidneys' ability to reabsorb bicarbonate (HCO3-) ions, which helps to reduce the acidity of the blood. However, the kidneys also increase the excretion of hydrogen ions by promoting the formation of carbonic acid (H2CO3) from carbon dioxide (CO2) and water (H2O). The carbonic acid then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). As a result, the kidneys increase the excretion of CO2 as well, which leads to a decrease in PCO2 levels in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the relationship between metabolic acidosis and PCO2 levels. In metabolic acidosis, the kidneys increase the excretion of CO2, leading to a decrease in PCO2 levels.

**Option B:** This option is incorrect because it suggests that PCO2 levels increase in metabolic acidosis, which is the opposite of what actually occurs.

**Option C:** This option is incorrect because it does not provide a clear explanation for the relationship between metabolic acidosis and PCO2 levels.

**Clinical Pearl / High-Yield Fact**
In metabolic acidosis, the kidneys try to compensate by increasing the excretion of hydrogen ions, which leads to a decrease in PCO2 levels. This is an important concept to remember when evaluating patients with metabolic acidosis.

**Correct Answer:** B. PCO2 levels decrease in metabolic acidosis.