Question:hard
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:
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:
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Low bicarbonate plus low pCO2: identify which one is the primary change and which is compensation.
Updated On: Jun 23, 2026
- K+ excretion by the kidney
- CO2 expiration by the lungs
- H+ excretion by the kidney
- HCO3 loss by the kidney
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The Correct Option is D
Solution and Explanation
Step-wise ABG reading angle: always name the disorder first, then separate cause from compensation.
The pH of 7.2 tells you the blood is acidemic. The bicarbonate is low (10 to 12), so this is a metabolic acidosis, since a primary fall in HCO3 lowers pH. Confirm with the pCO2: it is 35, a little below normal. A low pCO2 raises pH, so the lungs are compensating by hyperventilating. That makes option 2 a description of compensation, not the root cause, so it is out.
Now decide the primary mechanism. The defining abnormality is the dropped bicarbonate. Of the choices, renal bicarbonate loss (as occurs in proximal/type 2 renal tubular acidosis) directly produces a low HCO3 and this exact picture. Reduced renal H+ excretion causes acidosis too, but the option framed as a bicarbonate-lowering loss fits the low HCO3 best. Potassium handling is not the driver here.
So the metabolic acidosis here is best explained by bicarbonate loss by the kidney.
Ref: Ganong's Review of Medical Physiology, 26e.
The pH of 7.2 tells you the blood is acidemic. The bicarbonate is low (10 to 12), so this is a metabolic acidosis, since a primary fall in HCO3 lowers pH. Confirm with the pCO2: it is 35, a little below normal. A low pCO2 raises pH, so the lungs are compensating by hyperventilating. That makes option 2 a description of compensation, not the root cause, so it is out.
Now decide the primary mechanism. The defining abnormality is the dropped bicarbonate. Of the choices, renal bicarbonate loss (as occurs in proximal/type 2 renal tubular acidosis) directly produces a low HCO3 and this exact picture. Reduced renal H+ excretion causes acidosis too, but the option framed as a bicarbonate-lowering loss fits the low HCO3 best. Potassium handling is not the driver here.
So the metabolic acidosis here is best explained by bicarbonate loss by the kidney.
Ref: Ganong's Review of Medical Physiology, 26e.
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