## **Core Concept**
The anion gap is a calculated measure that helps in the diagnosis of metabolic acidosis and other conditions. It represents the difference between the concentrations of major cations and anions in the plasma. The main contributors to the anion gap are ions that are not routinely measured in a basic metabolic panel, such as phosphate, sulfate, and organic anions.

## **Why the Correct Answer is Right**
The anion gap is primarily due to **phosphate** and other organic anions, but among the given options, **chloride** is not the correct answer because it's a measured anion. The correct answer, **D.**, likely corresponds to an option that mentions unmeasured anions. Normally, chloride and bicarbonate are the measured anions. Unmeasured anions, which include phosphate, sulfate, and various organic anions, account for the majority of the anion gap. This concept is crucial in understanding acid-base balance and diagnosing metabolic acidosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent the primary contributor to the anion gap.
- **Option B:** This is incorrect for similar reasons; it likely represents a measured anion or an incorrect choice.
- **Option C:** This option is also incorrect as it does not correctly identify the main reason for the anion gap.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the anion gap can be estimated using the formula: Anion Gap = [Na+] - ([Cl-] + [HCO3-]). A normal anion gap ranges from 3 to 12 mEq/L. An increased anion gap suggests the presence of unmeasured anions, which is often seen in conditions like diabetic ketoacidosis, lactic acidosis, and renal failure.

## **Correct Answer:** D. Unmeasured anions (or a similar description).