Which statement about SIADH and urine gravity is true?

Which statement about SIADH and urine gravity is true?

• A. SIADH always lowers urine gravity
• B. SIADH causes concentrated urine with high gravity
• C. SIADH only affects plasma osmolality, not urine gravity
• D. SIADH does not affect urine gravity

Answer: (B)

In SIADH, excess ADH makes the kidneys reabsorb more water in the collecting ducts. This reduces serum osmolality and leads to hyponatremia, but the urine remains inappropriately concentrated because water is being reabsorbed. So the urine has a high osmolality and a higher specific gravity. That’s why the true statement is that SIADH causes concentrated urine with high gravity. The other options contradict the key effect of excessive ADH on the urine: SIADH does not produce dilute urine, and it affects both plasma and urine osmolality, not just plasma osmolality, and it does change urine gravity rather than leaving it unchanged.

In SIADH, excess ADH makes the kidneys reabsorb more water in the collecting ducts. This reduces serum osmolality and leads to hyponatremia, but the urine remains inappropriately concentrated because water is being reabsorbed. So the urine has a high osmolality and a higher specific gravity. That’s why the true statement is that SIADH causes concentrated urine with high gravity. The other options contradict the key effect of excessive ADH on the urine: SIADH does not produce dilute urine, and it affects both plasma and urine osmolality, not just plasma osmolality, and it does change urine gravity rather than leaving it unchanged.