Question

Low serum sodium with normal serum osmolality is a feature of

• A. Primary polydipsia
• B. Diuretic use
• C. Hyperlipidemia
• D. Hypoaldosteronism

Hint:

Think about which option is a lab artifact rather than a true body water or sodium problem.

Answer 1

The Correct Option is C

This question is about sorting the causes of low serum sodium by what happens to the serum osmolality at the same time. That single detail, a normal osmolality, points to one specific mechanism.

1. Primary polydipsia: drinking too much water dilutes the blood, so both the sodium level and the measured osmolality drop together. Osmolality is low here, not normal, so this does not fit.
2. Diuretic use: diuretics, especially thiazides, cause the kidney to lose sodium and impair urine dilution, again giving a low osmolality alongside the low sodium.
3. Hyperlipidemia: when triglycerides or cholesterol are very high, the fat takes up a large share of the plasma volume even though it carries no sodium. The lab test that measures sodium per volume of whole plasma is fooled into reporting a lower number, while the true sodium concentration in the water part of plasma, and the osmolality measured directly by freezing point, stay normal. This is called pseudohyponatremia.
4. Hypoaldosteronism: without enough aldosterone the kidney cannot hold on to sodium, so real sodium is lost from the body and osmolality falls along with it.

Only hyperlipidemia keeps the osmolality normal while the reported sodium looks low, because the problem is in the measurement, not in the body's actual water or salt balance. The other three are genuine hypo-osmolar states.

Let's summarize:

• Low sodium with low osmolality: primary polydipsia, diuretics, hypoaldosteronism (true hyponatremia).
• Low sodium with normal osmolality: hyperlipidemia or hyperproteinemia (pseudohyponatremia, a lab artifact).

So the condition that fits low sodium with normal osmolality is hyperlipidemia, and that is the correct answer here, even though this paper's original key lists a different option.