Finasteride EP Impurity A ; Finasteride USP RC A ; Finasteride Dihydro Impurity ; 1,2-Dihydro Finasteride Impurity ; (5?,17?)-N-(1,1-Dimethylethyl)-3-oxo-4-aza-androstane-17-carboxamide ; 1,2-Dihydro Finasteride ;