**Core Concept**
Respiratory alkalosis is a condition characterized by an elevated arterial pH and decreased carbon dioxide partial pressure (Pco2) due to hyperventilation. Partially compensated respiratory alkalosis is a state where the pH has returned to normal, but the Pco2 remains low, and bicarbonate (HCO3-) levels are still elevated in an attempt to compensate for the alkalosis.

**Why the Correct Answer is Right**
Option B describes a patient with partially compensated respiratory alkalosis after 1 month on a mechanical ventilator. The arterial pH is 7.50, indicating a slightly alkalotic state. The HCO3- level is elevated at 15 mEq/L, suggesting a compensatory response to the respiratory alkalosis. The Pco2 is decreased at 20 mm Hg, consistent with hyperventilation caused by the mechanical ventilator. As the body attempts to compensate for the alkalosis, the kidneys retain more HCO3- ions, increasing its levels in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the pH is elevated at 7.65, indicating a more severe alkalotic state than partial compensation. The HCO3- level is also elevated, but the Pco2 is not significantly decreased, making this option inconsistent with respiratory alkalosis.
**Option C:** Incorrect because the pH is normal at 7.40, but the HCO3- level is elevated, suggesting an attempt to compensate for a chronic respiratory alkalosis. The Pco2 is also normal, which is not consistent with the expected decrease in respiratory alkalosis.
**Option D:** Incorrect because the pH is decreased at 7.32, indicating acidosis, not alkalosis. The HCO3- level is also elevated, but this is not consistent with the expected decrease in respiratory alkalosis.
**Option E:** Incorrect because the pH is decreased at 7.31, indicating acidosis, not alkalosis. The HCO3- level is elevated, but the Pco2 is not significantly decreased, making this option inconsistent with respiratory alkalosis.

**Clinical Pearl / High-Yield Fact**
When interpreting arterial blood gas (ABG) results, remember that a decrease in Pco2 is a hallmark of respiratory alkalosis. The kidneys attempt to compensate for the alkalosis by retaining more HCO3- ions, increasing its levels in the blood. This compensatory response can be seen in the HCO3- level, which may remain elevated even after the Pco2 has returned to normal.

**Correct Answer: B. 7.50, 15, 20**