Express Scripts and Medco Health Solutions, two of the nation's largest pharmacy benefit managers, said Monday that federal regulators had approved their $29 billion merger. The decision, by the Federal Trade Commission, to let the merger proceed was not unanimous, indicating conflicting views among the agency's top regulators over whether to challenge—or impose limitations on—the combined company. After eight months of review, the F.T.C. commisioners voted 3-to-1 to close the agency's investigation. The acquisition of Medco by Express Scripts, based in St. Louis, creates what is now the industry's largest player, with $116 billion in 2011 revenue.