UnitedHealth Group Inc.'s fourth-quarter profit rose 21%, helped by growing health-insurance membership, signs of light health-care usage in some areas and fast-rising sales in the company's Optum health-services unit. The Minneapolis-based company, the largest managed-care concern by revenue, capped a strong year helped by a continued trend of muted patient traffic in hospitals and doctors' offices. Still, UnitedHealth held its 2012 guidance in check while maintaining a guarded outlook for the new year. The company anticipates patients will ramp up medical-system use, and it is making big investments related to the U.S. health-care overhaul law and changes in its pharmacy-benefits business, among other pressure points.