Mesa Airlines flight 6048 encountered turbulence at 10,000 ft during descent into Dulles International Airport (IAD), Chantilly, Virgina. As a result of the turbulence, one flight attendant sustained serious injuries. The airplane landed without further incident. The flight originated from LaGuardia International Airport (LGA), New York, New York. According to the flight crew, while descending on the HYPER EIGHT arrival, the air traffic controller advised their airplane was following a “heavy jet” and asked if a 7-mile separation would be sufficient. The captain asked for a 10-mile separation, which was granted, and the controller provided vectors to increase the spacing between the two airplanes. The crew could not see the other airplane visually, nor could they determine its location using the traffic collision avoidance system. During the descent, as the airplane crossed the HYPER intersection at 10,000 ft and 250 knots, it encountered moderate turbulence described as a “rapid jolt” while in instrument meteorological conditions. At the time, the seatbelt sign was on, and the two flight attendants along with another commuting flight attendant, were standing in the aft galley. The turbulence resulted in all three of them being “tossed about and hit the floor.” One flight attendant sustained a broken ankle. The flight crew declared a medical emergency and notified dispatch of the event. The commuting flight attendant assumed the duties for the injured flight attendant. The remainder of the fight was uneventful, and paramedics met the airplane at the gate and transported the injured flight attendant to the hospital. According to the operator, the flight crew was not aware of any forecast turbulence in the area or reports of turbulence encounters by other pilots (PIREPS).