Component of the microsomal membrane bound fatty acid elongation system, which produces the 26-carbon very long-chain fatty acids (VLCFA) from palmitate. Catalyzes the reduction of the 3-ketoacyl-CoA intermediate that is formed in each cycle of fatty acid elongation. VLCFAs serve as precursors for ceramide and sphingolipids. a very-long-chain (3R)-3-hydroxyacyl-CoA + NADP(+) = a very-long-chain 3-oxoacyl-CoA + H(+) + NADPH Lipid metabolism; fatty acid biosynthesis. Belongs to the short-chain dehydrogenases/reductases (SDR) family.