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