Final answer:
Demeclocycline is the medication used to treat SIADH resistant to fluid restriction because it inhibits the action of antidiuretic hormone, leading to increased excretion of free water and correction of dilutional hyponatremia.
Step-by-step explanation:
The medication used to treat SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion) that is resistant to fluid restriction is a) Demeclocycline. Demeclocycline is a tetracycline antibiotic which acts as a diuretic in the case of SIADH by inhibiting the action of the antidiuretic hormone (ADH), also known as vasopressin. Unlike drugs such as vasopressin or desmopressin which either mimic or enhance ADH's effect, demeclocycline allows for increased excretion of free water. This is particularly beneficial in SIADH, where there is an abnormal release of ADH leading to water retention and dilutional hyponatremia. Moreover, examples of diuretics like furosemide primarily work on the loop of Henle to inhibit the reabsorption of Na* and Cl-, which can also aid in the management of SIADH by reducing water reabsorption, although they are not the primary choice for SIADH resistant to fluid restriction.