Which statement about osmolarity and kidney response is true?

• A. Osmolarity increases when water is gained faster than sodium
• B. Osmolarity affects only urine volume
• C. When water loss exceeds sodium loss, kidneys conserve water
• D. Osmolarity is unaffected by dehydration

Answer: (C)

Osmolarity is the concentration of solutes in body fluids, so when water is lost relative to solute (water loss exceeding sodium loss), the fluids become more concentrated and osmolarity rises. The body detects this change and signals the kidneys to conserve water. This happens via the hypothalamus triggering antidiuretic hormone (ADH) release, which increases water reabsorption in the collecting ducts through aquaporin channels. The result is less, but more concentrated urine, helping to restore osmolar balance by retaining water.

The other statements don’t fit because increasing water intake relative to solute would lower osmolarity, osmolarity influences more than just urine volume (it governs urine concentration and water reabsorption), and dehydration does affect osmolarity rather than leaving it unchanged.