Ans: B. Metabolic acidosis(Ref: Harrison 19/e p317, 18Ie p365)In the question, pH is decreased (acidosis) & pCO, is decreased (Normal= 40-45 mm Hg).A decreased pCO2 will try to increase pH, hence it must be secondary compensating mechanism.So, the primary mechanism causing the acid base imbalance must be a decrease in serum bicarbonate concentration i.e. metabolic acidosis.