A 2 year old child is being evaluated for persistent metabolic acidosis. Blood tests show Na+ 140 mEq/I, K+ 3 mEq/I, Ca2+ 8 mg/dL, mg2+2 mg/dL, phosphate 3 mg/dl, pH 7.22, bicarbonate 16 mEq/I and chloride 112 mEq/I. The plasma anion gap is -