This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Renal physiology, Subject name: Physiology
In a patient pO2 is 85 mmHg, pCO2 is 50 mmHg, pH is 7.2, and HCO3- is 36mmol/L. The condition is 
Select and explain. 
A. Respiratory acidosis with compensatory metabolic alkalosis, B. Uncompensated respiratory acidosis, C. Respiratory alkalosis with compensatory metabolic acidosis, D. Respiratory acidosis with compensatory metabolic acidosis
A: Respiratory acidosis with compensatory metabolic alkalosis
Answer: A: Respiratory acidosis with compensatory metabolic alkalosis 
Explanation: pH is decreased (<7.35) so it is acidosis. The CO2 levels are increased (>45 mmHg), which is in line with the change in pH (in acidosis CO2 is expected to raise), thus the primary problem is respiratory. The HCO3- levels are increased (>30mmol/L), which is opposite to change in pH (i.e., there is acidosis and HCO3- levels are high), thus , there is compensatory metabolic alkalosis. Thus this is a case of respiratory acidosis with compensatory metabolic alkalosis. Ref: Oxford handbook of medicine 5th edition Pgno: 684