Which of the following medications is used for the treatment of SIADH?

Demeclocycline is commonly used in the management of SIADH (Syndrome of Inappropriate Antidiuretic Hormone secretion). This medication is a tetracycline antibiotic that has been found to induce nephrogenic diabetes insipidus, which leads to a reduction in the kidney's responsiveness to antidiuretic hormone (ADH). By decreasing the effects of ADH on the kidney, demeclocycline promotes diuresis and thus helps to manage the fluid retention associated with SIADH.

Other medications mentioned, such as hydroxyurea, phenytoin, and sublingual nitroglycerin, do not have a direct role in treating SIADH. Hydroxyurea is mainly used in certain hematological conditions, phenytoin is primarily an anticonvulsant, and sublingual nitroglycerin is an agent used for angina due to its vasodilatory properties, but none of them address the underlying fluid and electrolyte imbalance occurring in SIADH. Therefore, demeclocycline is the most appropriate choice for the treatment of this condition.