A 2 year old child is being evaluated for persistent metabolic acidosis. Blood tests show Na+ 140mEq/1. K+ 3mEq/1, Ca2+ 8mg/1, Mg+2 mEq/l. phosphate 3 mEq/1, pH 7.22, bicarbonate 16mEq/1 and chloride 112mEq/1. 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 blood. The formula for calculating 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:** If we calculate the anion gap as less than 12 (for example, option A is 8), it would be too low and not reflective of the patient's calculated anion gap.
- **Option B:** Similarly, if the anion gap were significantly higher (for example, option B), it would not match our calculated value of 12.
- **Option D:** If the anion gap were given as a value different from 12 (for example, option D), it would also not match our calculation.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that an elevated anion gap (>12 mEq/L) often indicates the presence of ketoacidosis (as in diabetic ketoacidosis), lactic acidosis, or ingestion of certain toxins. A normal anion gap metabolic acidosis, on the other hand, often points towards a gastrointestinal or renal cause of bicarbonate loss.

## **Correct Answer:** . 12.