In a patient with low HCO3 and low pCO2, what is the next test, and what are the potential differential diagnoses?
A) Urine pH; Renal tubular acidosis (RTA)
B) Serum bicarbonate; Metabolic acidosis
C) Arterial blood gas (ABG); Respiratory alkalosis
D) Serum anion gap; Diabetic ketoacidosis