Catalyzes the irreversible stereospecific reduction of the delta 4,5 bond (double bond between carbons 4 and 5) of various 3-oxo steroids (3-keto steroids) producing their 5alpha dihydro-3-oxo forms (PubMed:10898110, PubMed:23685396). Converts testosterone into 5-alpha- dihydrotestosterone (DHT), the most active androgen in the prostate, as it is the preferred ligand for androgen receptor (AR) transactivation, making this reaction a key step in male sexual differentiation during development (PubMed:10898110, PubMed:23685396). Besides testosterone, it can also act on other steroids, including progesterone, producing metabolites with diverse roles (PubMed:10898110). Hence, it plays a central role in sexual differentiation and androgen physiology (PubMed:10898110). {ECO:0000269|PubMed:10898110, ECO:0000269|PubMed:23685396}. This is the function of ENSG00000277893 (SRD5A2, steroid 5 alpha-reductase 2).