This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fluids and Electrolyte Imbalance, Subject name: Medicine
A 70 year old man with history of CHF presents with increased shoness of breath and leg swelling. ABG: pH 7.24, pC02 = 60 mmHg, P02 = 52, HC03 =27. Interpret: 
Select and explain. 
A. Metabolic acidosis, B. Metabolic alkalosis, C. Respiratory acidosis, D. Respiratory alkalosis
C: Respiratory acidosis
Answer: C: Respiratory acidosis 
Explanation: If there is acute respiratory acidosis, expected compensation is rise of HCO3 by 1 mEq/L for every 10 mm Hg rise of pCO2. In this question change in pCO2 = 60 -40 = 20mmHg Therefore expected elevation in = 2 m Eq. Since the actual HCO3- is close to the calculated HCO3, the diagnosis is respiratory acidosis