Catalyzes the NADPH-dependent reduction of D-ribulose 5-phosphate to D-ribitol 5-phosphate and the further reaction of D-ribitol 5-phosphate with CTP to form CDP-ribitol. CTP + D-ribitol 5-phosphate + H(+) = CDP-L-ribitol + diphosphate D-ribitol 5-phosphate + NADP(+) = D-ribulose 5-phosphate + H(+) + NADPH kcat is 22 sec(-1) for the reductase reaction. kcat is 13 sec(-1) for the cytidyltransferase reaction. Capsule biogenesis; capsule polysaccharide biosynthesis. Monomer. In the N-terminal section; belongs to the IspD/TarI cytidylyltransferase family. In the C-terminal section; belongs to the short-chain dehydrogenases/reductases (SDR) family.