Which drug is an example of a 5-alpha reductase inhibitor?

Finasteride (Proscar) is classified as a 5-alpha reductase inhibitor, a type of medication that is commonly used to treat benign prostatic hyperplasia (BPH) and sometimes male pattern hair loss. The drug works by inhibiting the enzyme 5-alpha-reductase, which is responsible for converting testosterone into dihydrotestosterone (DHT). Since DHT is a crucial factor in prostate growth and hair loss, reducing its levels can help shrink the prostate and improve urinary symptoms in patients with BPH, as well as promote hair regrowth in individuals dealing with androgenetic alopecia.

The other medications listed do not fall into this class; terazosin and tamsulosin are alpha-1 adrenergic blockers that help relax the muscles in the prostate and bladder neck, thus improving urine flow, but they do not influence the hormonal pathways like finasteride. Sildenafil, on the other hand, is a phosphodiesterase type 5 (PDE5) inhibitor used primarily to treat erectile dysfunction, with a completely different mechanism of action unrelated to prostate size or hair loss.