**Question:** In a 32-year-old male presenting with the following blood chemistry: Na+ → 135, K+ → 5.0, HCO3- → 14.0, Cl- → 116, PO4-- → 5.0, SO4-- → 5.0, Mg++ → 2.0, Ca++ → 8.0. The anion gap is:

A. 10
B. 10.5
C. 14
D. 14.5

**Correct Answer:** D. 14.5

**Core Concept:** Anion gap is a measure of the difference between the anion concentration in blood and the calculated anion concentration based on measured anions (chloride, phosphate, and sulfate). A high anion gap indicates the presence of unmeasured anions, which may be due to acidosis, ketoacidosis, or other unmeasured anions.

**Why the Correct Answer is Right:** In this case, the patient has the following measured anions: chloride (Cl-) 116 mmol/L, phosphate (PO4-) 5 mmol/L, and sulfate (SO4-) 5 mmol/L. The calculated anion concentration would be:

(116 mmol/L + 5 mmol/L + 5 mmol/L) / 2 = 30 mmol/L

The anion gap is calculated by subtracting the calculated anion concentration from the measured anion concentration:

Anion Gap = Measured Anion Concentration - Calculated Anion Concentration
Anion Gap = 116 mmol/L - 30 mmol/L = 86 mmol/L

**Why Each Wrong Option is Incorrect:**

A) The anion gap is not calculated correctly. The correct calculation is mentioned above.

B) The anion gap is too high (10.5 mmol/L), which would be expected in severe ketoacidosis or lactic acidosis, but the patient's acid base status does not suggest these conditions.

C) The anion gap is not calculated correctly due to the same reason as Option A.

D) The calculated anion gap (30 mmol/L) is too low, indicating the presence of unmeasured anions. In this case, the measured anion gap (86 mmol/L) is higher than the calculated anion gap (30 mmol/L), suggesting the presence of unmeasured anions. The high anion gap is likely due to the presence of unmeasured anions, which are usually found in the context of ketoacidosis or lactic acidosis. However, in this case, the patient does not exhibit severe acidosis or have elevated blood ketones or lactate. Instead, the high anion gap is indicative of the presence of unmeasured anions, which are typically found in the context of:

1. **Hypokalemic alkalosis**
2. **Acidosis due to potassium loss**
3. **Acidosis