Which expression represents the anion gap when using sodium, potassium, bicarbonate, and chloride?

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The concept being tested is how to calculate the anion gap using the major measured ions. The anion gap represents the difference between the measured cations and the measured anions in serum. When potassium is part of the data, include it on the cation side: take the sum of the measured cations (sodium plus potassium) and subtract the sum of the measured anions (chloride plus bicarbonate).

So the correct expression is Na + K minus (HCO3 plus Cl). This reflects the idea that the gap accounts for unmeasured anions by balancing the positively charged ions against the negatively charged ions.

If you omit potassium and use Na minus (HCO3 plus Cl), you’re using a version that assumes potassium is negligible, which isn’t appropriate when potassium is provided. The other options either place chloride on the wrong side or sum everything together, which does not compute a gap.