Which of the following is NOT one of the muscarinic antagonist drugs used to treat overactive bladder?

The correct choice is Hydrochlorothiazide, as it uniquely serves a different purpose in medical treatment. Hydrochlorothiazide is primarily a thiazide diuretic used to manage hypertension and edema by promoting the excretion of sodium and water from the body. It does not possess anticholinergic properties nor is it indicated for treating overactive bladder.

In contrast, Darifenacin, Solifenacin, and Oxybutynin are all muscarinic antagonists specifically developed for overactive bladder management. These medications work by blocking muscarinic receptors in the bladder, thereby reducing involuntary contractions and the urgency associated with overactive bladder symptoms. Each of these anticholinergic agents contributes to improving bladder function and increasing the capacity of the bladder, which is essential for patients experiencing such conditions.