A patient with severe diarrhea has a blood pH of 7.2, a bicarbonate level of 12 mmol/L, and a PCO2 of 40 mmHg. What is the most likely acid-base disturbance in this patient?

The patient's low blood pH and low bicarbonate level indicate metabolic acidosis. The normal PCO2 level suggests that there is some degree of respiratory compensation, but it is not sufficient to fully correct the acidosis.