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Alpha 1 antagonists are specifically designed to block the alpha-1 adrenergic receptors, which are present in the smooth muscle of the bladder neck and prostate. By doing so, these medications promote relaxation of the smooth muscle tissue in these areas. This relaxation leads to a reduction in resistance to urinary flow, thereby alleviating the symptoms associated with benign prostatic hyperplasia (BPH), such as difficulty in urination, incomplete bladder emptying, and decreased urinary stream.

The action of relaxing smooth muscle is particularly beneficial in patients suffering from BPH as it directly addresses the mechanical obstruction that results from the enlarged prostate. The effectiveness of these medications in improving urinary symptoms highlights their role in managing the condition.

Other options do not accurately represent the mechanism of action of alpha 1 antagonists. For example, while decreasing urinary frequency may be a result of symptom relief, it is not a direct mechanism of action. Similarly, inhibiting testosterone production is not part of how alpha 1 antagonists work, and increasing bladder capacity is not a direct effect of these drugs. Thus, the focus remains on the relaxation of smooth muscle as the primary mechanism for treating BPH.