Which statement best describes the primary abnormality in respiratory acidosis?

• A. Hypoventilation leading to increased PCO2
• B. Hyperventilation leading to decreased PCO2
• C. Increased pH with decreased PCO2
• D. Decreased bicarbonate with decreased PCO2

Answer: (A)

Respiratory acidosis is driven by elevated CO2 from inadequate ventilation. When ventilation falls short, carbon dioxide builds up in the blood, combines with water to form carbonic acid, and increases hydrogen ion concentration, lowering the pH. The hallmark is a high arterial PCO2, which directly causes the acidemia seen in this condition. In acute cases the key change is the rise in CO2; in chronic cases the kidneys may compensate by retaining bicarbonate, but the root problem remains CO2 retention from hypoventilation.

The other descriptions describe different patterns: hyperventilation lowers CO2 and causes respiratory alkalosis, not acidosis; a scenario with decreased pH and decreased CO2 would not fit respiratory acidosis; and a situation with increased pH and decreased CO2 would likewise indicate alkalosis.