Final answer:
The action of a 5 alpha-reductase inhibitor in the treatment of BPH is to inhibit the conversion of testosterone to dihydrotestosterone (DHT), reducing prostate size and improving urinary symptoms.
Step-by-step explanation:
The action of a 5 alpha-reductase inhibitor in the treatment of BPH is to inhibit the conversion of testosterone to dihydrotestosterone (DHT). DHT is a hormone that contributes to the growth of prostate tissue. By blocking this conversion, 5 alpha-reductase inhibitors can help to reduce the size of the prostate gland and alleviate urinary symptoms associated with BPH. These inhibitors do not block the effects of adrenaline, relax smooth muscles in the bladder, nor increase the production of prostatic acid phosphatase as might be suggested by other options.