Question

An arterial blood gas shows pH 7.2, HCO3 10 to 12 mmol/L and pCO2 35 mm Hg. This metabolic acidosis is due to:

• A. K+ excretion by the kidney
• B. CO2 expiration by the lungs
• C. H+ excretion by the kidney
• D. HCO3 loss by the kidney

Hint:

Low bicarbonate plus low pCO2: identify which one is the primary change and which is compensation.

Answer 1

The Correct Option is D

Answer: HCO3 loss by the kidney.
Step 1: Read the gas. pH 7.2 is acidemia. HCO3 is low at 10 to 12 mmol/L (normal about 22 to 26), so the primary problem is a metabolic acidosis.
Step 2: The pCO2 is 35 mm Hg, below the normal 40, so the lungs are blowing off CO2 to try to raise the pH. That low pCO2 is the expected RESPIRATORY COMPENSATION, not the cause. So option 2 (CO2 expiration) describes compensation, not the primary defect.
Step 3: The primary defect is the fall in bicarbonate. Among the options, loss of bicarbonate (here renal HCO3 wasting, as in type 2 proximal renal tubular acidosis) directly lowers HCO3 and produces this picture. Failure to excrete H+ would also cause acidosis, but the cause stated as a bicarbonate-lowering process best matches the low HCO3 given. K+ excretion does not by itself drive this acidosis.
Hence bicarbonate loss is the answer.
Ref: Ganong's Review of Medical Physiology, 26e.