## **Core Concept**
The given question involves interpreting arterial blood gas (ABG) values to determine the acid-base status of a patient. ABG analysis includes pH, PaCO2 (partial pressure of carbon dioxide), and HCO3- (bicarbonate) levels, which help in diagnosing acid-base disorders.

## **Why the Correct Answer is Right**
To interpret the given ABG values: PaCO2 - 65 mmHg, HCO3- - 15 mEq/L, and pH - 7.21, let's analyze them step by step:
- The pH is 7.21, which is acidic.
- The PaCO2 is 65 mmHg, which is elevated, indicating respiratory acidosis.
- The HCO3- is 15 mEq/L, which is decreased.

Given these values, the primary disorder is respiratory acidosis (elevated PaCO2). The HCO3- level is also decreased, which could indicate a secondary response to the respiratory acidosis or a mixed disorder. However, the decrease in HCO3- is not sufficient to compensate for the elevated PaCO2, and the pH remains acidic.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option would be incorrect if it suggested a different primary diagnosis that didn't align with the provided ABG values, such as metabolic acidosis without considering the respiratory component.
- **Option B:** If this option suggested a respiratory alkalosis or a compensated metabolic disorder without acknowledging the acidosis, it would be incorrect.
- **Option C:** If this option did not accurately reflect a diagnosis of respiratory acidosis with or without a metabolic component based on the provided ABG values, it would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that in respiratory acidosis, the kidneys try to compensate by increasing HCO3- reabsorption. However, this compensatory mechanism takes time (hours to days). An ABG can help differentiate between acute and chronic respiratory acidosis by assessing the level of HCO3- compensation.

## **Correct Answer:** . Respiratory Acidosis.