United Airlines flight 1759 encountered moderate turbulence while descending through 11,000 ft for landing at Cincinnati/Northern Kentucky International Airport (CVG) Hebron, Kentucky, and a flight attendant (FA) fractured her pelvis. The flight crew reported that while descending on the SARGO-3 arrival into CVG they observed a single cloud with no apparent vertical development at about 11,000 ft. With no indication of an adverse ride from air traffic control (ATC), their onboard weather applications or their weather radar, the pilots elected to continue. Upon entering the cloud, the flight encountered about 2 seconds of moderate turbulence. Following the turbulence encounter the flight crew received a call from the cabin advising them that a FA had been injured. The aft FA assigned to jumpseat 2L reported that she was completing her duties in the aft galley and was seated in her jumpseat but had not fastened her safety harness, when suddenly and without warning, they encountered severe turbulence. She was thrown into the air, struck the ceiling and impacted the floor on her side fracturing her pelvis. Upon being notified of the injury, the flight crew declared a medical emergency and requested paramedics meet the aircraft at the gate. The injured FA was transported to the hospital where she was diagnosed with a fractured pelvis. At the time of the turbulence encounter there were no active significant meteorological information (SIGMETs), convective SIGMETs, airman’s meteorological information (AIRMETs), or pilot reports (PIREPs) of any significant turbulence.