With increased water loss, which electrolyte level is most likely elevated in serum?

• A. Sodium
• B. Potassium
• C. Magnesium
• D. Calcium

Answer: (A)

When water is lost without a proportional loss of solutes, the extracellular fluid becomes more concentrated. Sodium is the main extracellular solute and largely sets serum osmolality, so its concentration increases as water is depleted. This leads to hypernatremia, a common consequence of dehydration or other conditions causing free water loss. The kidneys may conserve water, but the relative loss of water than salt drives the serum sodium up.

Potassium, magnesium, and calcium levels are influenced by different regulatory factors—renal handling, acid-base balance, hormonal control, and bone or cellular processes—so they don’t rise in the same straightforward way with pure water loss.