Enzyme converting testosterone into DHT:
Development of male external genitalia requires local conversion of testosterone secreted by Leydig cells into more active androgen dihydrotestosterone. Reaction catalyzed by microsomal enzyme steroid 5α-reductase type 2 encoded by SRD5A2 gene which uses NADPH to reduce Δ4-5 double bond. DHT exhibits higher affinity for androgen receptor and prolonged receptor-DNA interaction, activating genes for penile growth, urethral closure and scrotal fusion. Deficiency of enzyme causes pseudovaginal perineoscrotal hypospadias despite normal Wolffian ducts, while aromatase converts testosterone to estradiol irrelevant for this masculinization.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 17: 5α-reductase converts testosterone to DHT.