Which statement best differentiates diabetes insipidus from SIADH?

Which statement best differentiates diabetes insipidus from SIADH?

• A. Diabetes insipidus often presents with edema and hyponatremia, SIADH with polyuria
• B. Both conditions present with high urine osmolality
• C. Diabetes insipidus presents with polyuria and hypernatremia; SIADH presents with hyponatremia and fluid overload
• D. SIADH causes dehydration and hyperkalemia

Answer: (C)

The key idea is that these conditions produce opposite water balance and sodium disturbances due to ADH effects. Diabetes insipidus results from a lack of ADH action, so the kidneys can’t concentrate urine. This leads to large volumes of dilute urine (polyuria) and a rise in serum sodium (hypernatremia) due to water loss. In contrast, SIADH involves excess ADH, causing water retention, dilution of serum sodium (hyponatremia), and usually a euvolemic or mildly fluid-overloaded state rather than dehydration. So the statement that best differentiates them is that diabetes insipidus presents with polyuria and hypernatremia, while SIADH presents with hyponatremia and fluid overload (or euvolemia). Other choices don’t fit this opposite pattern: diabetes insipidus does not typically cause edema or hyponatremia, and it does not feature high urine osmolality; SIADH does not cause dehydration or hyperkalemia.

The key idea is that these conditions produce opposite water balance and sodium disturbances due to ADH effects. Diabetes insipidus results from a lack of ADH action, so the kidneys can’t concentrate urine. This leads to large volumes of dilute urine (polyuria) and a rise in serum sodium (hypernatremia) due to water loss. In contrast, SIADH involves excess ADH, causing water retention, dilution of serum sodium (hyponatremia), and usually a euvolemic or mildly fluid-overloaded state rather than dehydration.

So the statement that best differentiates them is that diabetes insipidus presents with polyuria and hypernatremia, while SIADH presents with hyponatremia and fluid overload (or euvolemia). Other choices don’t fit this opposite pattern: diabetes insipidus does not typically cause edema or hyponatremia, and it does not feature high urine osmolality; SIADH does not cause dehydration or hyperkalemia.