## 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, 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 most significantly, it is attributed to the presence of **albumin**, which is the most abundant unmeasured anion in the plasma. Albumin is a major contributor because it is present in high concentrations and has a significant charge at physiological pH.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because, although chloride is a measured anion, it does not contribute to the anion gap; rather, it is one of the ions used in the calculation to estimate the gap.
* **Option B:** This option is incorrect because, like chloride, bicarbonate is a measured anion and does not contribute to the anion gap.
* **Option D:** This option is incorrect because sodium is a measured cation, not an anion, and thus does not contribute to the anion gap.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the normal anion gap ranges from 8 to 12 mEq/L. An increased anion gap often indicates the presence of unmeasured anions, such as in diabetic ketoacidosis or lactic acidosis. A useful mnemonic to calculate the anion gap is: Anion Gap = [Na+] - ([Cl-] + [HCO3-]).

## Correct Answer: C.