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:

## **Core Concept**
The anion gap is a calculated measure used in the evaluation of metabolic acidosis. It represents the difference between the major measured cations (sodium) and anions (chloride and bicarbonate) in the plasma. The formula to calculate the anion gap is: Anion Gap = [Na+] - ([Cl-] + [HCO3-]).

## **Why the Correct Answer is Right**
To calculate the anion gap for this patient, we substitute the given values into the formula: Anion Gap = 140 - (112 + 16) = 140 - 128 = 12. The normal anion gap range is approximately 8-12 mEq/l. Therefore, an anion gap of 12 falls within the upper limit of normal, suggesting that this patient's metabolic acidosis could be due to a non-anion gap (or normal anion gap) metabolic acidosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Anion Gap of 8 would be lower than the calculated value, suggesting an error in calculation or provided values, which is not the case here.
- **Option B:** Anion Gap of 10 is close but does not match the calculated value of 12.
- **Option D:** Anion Gap of 15 would indicate an elevated anion gap metabolic acidosis, which does not align with the calculated value.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that a normal anion gap metabolic acidosis often points towards a gastrointestinal or renal cause of bicarbonate loss, or an issue with acid excretion. Causes include severe diarrhea, renal tubular acidosis, and certain types of kidney disease.

## **Correct Answer:** .