What is the most common extracellular anion?

• A. Chloride (Cl-)
• B. Bicarbonate
• C. Phosphate
• D. Sulfate

Answer: (A)

Chloride is the most common extracellular anion because the extracellular fluid’s negative charge balance is mainly maintained by chloride working with the dominant cation, sodium. In plasma and interstitial fluid, sodium is the primary extracellular cation, and chloride serves as the main counterion to keep charges balanced. Its normal concentration is around 100–105 mEq/L, which is higher than the other common extracellular anions. Bicarbonate is also present as part of the buffering system, but its extracellular level is much lower, about 24 mEq/L, and it varies with CO2 and acid–base status. Phosphate and sulfate exist in extracellular fluid at much lower levels and are more concentrated intracellularly, so they contribute less to the extracellular anion pool. Thus, chloride stands out as the most abundant extracellular anion.