An ABG showing respiratory acidosis may present with which of the following as the primary abnormality?

• A. PaCO2 50 mmHg and pH 7.32
• B. PaCO2 40 mmHg and pH 7.40
• C. HCO3- 24 mEq/L
• D. pH 7.50

Answer: (D)

Respiratory acidosis is defined by CO2 retention from hypoventilation, which raises the PaCO2 and drives the pH downward (acidemia). The primary abnormality you look for is an elevated PaCO2 with a low pH.

In these options, the pattern with a PaCO2 of 50 mmHg and a pH of 7.32 fits respiratory acidosis best: high CO2 plus acidic pH. The normal PaCO2 with normal pH would indicate no disturbance. A normal bicarbonate (HCO3- 24) doesn’t point to the primary issue here, and a pH of 7.50 indicates alkalemia, not acidosis, which would align with metabolic alkalosis or respiratory alkalosis depending on the CO2 level.

So the key concept is that increased CO2 with a decreased pH signals respiratory acidosis—the pattern shown by the CO2 50 mmHg and pH 7.32 option.