**Core Concept**
The partial pressure of carbon dioxide (PCO2) in expired air is an essential parameter in assessing respiratory function and acid-base balance. The body's ability to regulate PCO2 is crucial for maintaining proper pH levels in the blood.

**Why the Correct Answer is Right**
The partial pressure of carbon dioxide in expired air is a reflection of the body's metabolic production of CO2. When carbon dioxide is produced in the body's metabolic processes, it diffuses into the bloodstream, where it binds to hemoglobin and is transported to the lungs for exhalation. The partial pressure of CO2 in expired air represents the net effect of CO2 production, diffusion, and exhalation. In a normal individual, the partial pressure of CO2 in expired air is approximately 32 mm Hg.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.3 mm Hg is an implausibly low value for PCO2 in expired air, as it would indicate significant hyperventilation or respiratory alkalosis.

**Option B:** 158 mm Hg is an unrealistically high value for PCO2 in expired air, as it would indicate severe respiratory acidosis or a profound failure of the respiratory system.

**Option C:** 40 mm Hg is lower than the normal value for PCO2 in expired air, which would indicate hyperventilation or respiratory alkalosis.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation is that an elevated PCO2 (hypercapnia) is often seen in conditions such as chronic obstructive pulmonary disease (COPD), pneumonia, or other respiratory failures, whereas a decreased PCO2 (hypocapnia) is seen in conditions such as hyperventilation or severe asthma.

**✓ Correct Answer: D. 32 mm Hg**