What acid-base disturbance is characterized by a low pH and low HCO3− with a compensatory decrease in PaCO2?

• A. Respiratory acidosis
• B. Metabolic acidosis with appropriate respiratory compensation
• C. Metabolic alkalosis
• D. Mixed metabolic acidosis with respiratory alkalosis

Answer: (B)

Low pH with low bicarbonate points to metabolic acidosis. The body's response is to blow off CO2, lowering PaCO2, which helps raise the pH toward normal. When the measured PaCO2 falls in the expected range for the amount of bicarbonate loss, this is appropriate respiratory compensation for metabolic acidosis. Other patterns don’t fit: respiratory acidosis would have high PaCO2, metabolic alkalosis would raise both pH and HCO3-, and a mixed disorder (like metabolic acidosis with respiratory alkalosis) would not show a straightforward compensatory decrease in PaCO2.