Which medication relaxes the internal urethral sphincter and is used to treat BPH?

Prazosin, an alpha-adrenergic antagonist, is effective in treating benign prostatic hyperplasia (BPH) because it works by relaxing the smooth muscle in the internal urethral sphincter and prostatic urethra. This action allows for improved urinary flow and alleviates symptoms associated with BPH, such as difficulty urinating or increased frequency of urination. By antagonizing alpha-adrenergic receptors, prazosin reduces the tone of the smooth muscle that constricts the urethra, thereby easing the passage of urine.

Other medications listed serve different purposes: benzodiazepines like Librium are primarily used for anxiety and have no impact on the urinary tract; furosemide, a diuretic, helps to increase urine production but does not alleviate the obstruction caused by BPH; and amitriptyline, a tricyclic antidepressant, is used mainly for depression and may have anticholinergic effects that can worsen urinary retention in patients with BPH. Thus, prazosin is the most appropriate choice for addressing the specific therapeutic need related to BPH.