**Core Concept**
Respiratory acidosis is a form of acid-base disorder characterized by an elevated arterial carbon dioxide partial pressure (PaCO2) due to inadequate ventilation, leading to a decrease in blood pH.

**Why the Correct Answer is Right**
The patient presents with severe COPD, characterized by a significantly reduced FEV1, indicating severe airflow limitation. The recent increase in dyspnea, sputum production, and purulence suggests a likely exacerbation of his COPD. The arterial blood gas (ABG) shows a pH of 7.25, indicating acidosis, with a PaCO2 of 68, which is significantly elevated. This is consistent with respiratory acidosis due to hypoventilation in the context of severe COPD. The high PaCO2 level in this case is a direct result of the patient's inability to adequately ventilate, likely due to his severe airflow limitation.

**Why Each Wrong Option is Incorrect**

**Option A:** Respiratory alkalosis would be characterized by a low PaCO2 and high pH, which is not consistent with this patient's presentation.
**Option B:** Metabolic acidosis would be characterized by a low pH and high anion gap, but the patient's ABG shows a normal anion gap (not provided, but implied by the normal HCO3 level).
**Option C:** Metabolic alkalosis would be characterized by a high pH and low PaCO2, which is not consistent with this patient's presentation.

**Clinical Pearl / High-Yield Fact**
In patients with severe COPD, it is essential to manage respiratory acidosis promptly, as it can lead to further complications such as respiratory failure, cardiac arrhythmias, and even death.

**Correct Answer:** D. Respiratory acidosis