Southwest Airlines Flight 3633 (SWA 3633) encountered turbulence while descending through flight level (FL) 190 to Baltimore/Washington International Thurgood Marshall Airport (BWI), Baltimore, Maryland. The flight was a regularly scheduled domestic passenger flight from Manchester-Boston Regional Airport (MHT), Manchester, New Hampshire to BWI. The flight crew illuminated the seat belt sign during descent into BWI. A few minutes following the illumination, a passenger stood up and walked to the back of the cabin. The flight attendant told the passenger to return to her seat, and the passenger responded that she would not return to her seat because she needed to stretch. About a minute later, as the aircraft descended through a cloud, the flight encountered short duration moderate turbulence. The standing passenger lost her balance and fell to the floor, breaking her ankle. Two nurses came to assist the passenger on the floor of the galley where they bandaged and splinted her ankle. The nurses returned to their seats for landing. An off-duty flight attendant sat on the floor of the galley with the injured passenger to secure her during landing. EMTs met the flight at the gate and transported the injured passenger from the airplane to a local hospital. The flight crew reported that the there were no reports of turbulence or radar returns that would have indicated turbulence. A post flight review of weather showed no predicted turbulence or evidence of convection near the location and altitude where the encounter occurred. Probable Cause: A passenger’s failure to comply with crew member instructions to remain seated with seatbelt fastened.