The main cation in extracellular fluid (ECF) making up over 50% is:

Appeared in: PGICH Noida NO-2025

Explanation

Sodium (Na⁺) is the most abundant cation in the extracellular fluid (ECF), accounting for over 90% of the cations in this compartment.
Its high concentration in the ECF is crucial for maintaining osmotic pressure, which regulates water balance between the ECF and intracellular fluid (ICF).
The normal serum (ECF) sodium level is approximately 135-145 mEq/L, far exceeding the concentration of other cations like potassium, calcium, and magnesium in the ECF.

Option A: Potassium (K⁺) is the main cation of the intracellular fluid (ICF), not the extracellular fluid. Its concentration is much lower in the ECF (3.5-5.0 mEq/L) compared to inside the cells (around 140-150 mEq/L).
Option C: Magnesium (Mg²⁺) is the second most abundant cation in the intracellular fluid (ICF), after potassium. Only a very small fraction is found in the ECF.
Option D: Chloride (Cl⁻) is an anion (negatively charged ion), not a cation (positively charged ion). While it is the major anion in the ECF, the question specifically asks for the main cation.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Electrolyte distribution in body fluid compartments as background academic context rather than a clinical decision trigger.
Monitoring a patient's sodium level is a fundamental nursing responsibility, as imbalances (hyponatremia or hypernatremia) can lead to serious neurological complications, such as confusion, seizures, or coma.
Changes in ECF sodium concentration directly affect fluid shifts between compartments. For example, low sodium (hyponatremia) can cause water to move into brain cells, leading to cerebral edema.
Nurses must carefully manage intravenous (IV) fluid administration, as the type of fluid (e.g., isotonic, hypotonic, hypertonic) directly impacts a patient's sodium levels and fluid balance.

How to Approach the Question

First, identify the key terms in the question: 'main cation' and 'extracellular fluid (ECF)'.
Differentiate between a 'cation' (positive charge) and an 'anion' (negative charge). This allows you to immediately question the validity of 'Chloride' as an answer.
Recall the basic composition of the body's fluid compartments. Remember the mnemonic 'salt is outside' to associate Sodium (Na⁺) with the extracellular fluid.
Compare the roles of the remaining cations. Potassium (K⁺) is the primary intracellular cation.
Based on this, conclude that Sodium is the main extracellular cation.

Concept Tested & Keywords

Q_va4ptNLH00zXO1yFjBGw

E6 Nursing Fundamentals Taylor p. 773-775

E6 Nursing Brunner Adult Health 3SA Vol 1 Part 1 p. 134-136

E6 Pharmacology Nursing Lilley 11e Part 2 p. 164-166

Practise the full PGICH Noida NO-2025

Attempt every question from this paper in a timed mock, then review the full solution for each one.