A flight attendant was seriously injured when United Airlines flight 1734 encountered convectively-induced turbulence during the descent into the Chicago O'Hare International Airport (ORD), Chicago, Illinois. Flight 1734 originated from the San Francisco International Airport (SFO) and the captain was the pilot monitoring and the first officer was the pilot flying. The flight crewmembers reported that they were not aware of any adverse reports of turbulence from either air traffic control (ATC), or other aircraft and they found the route along the arrival path to be generally smooth. Their weather radar was on and showed some cells located to the west of ORD, but none were along either their arrival route, the airport itself, or the area east of ORD where they conducted their approach to landing. When they first checked in with the Chicago approach controller, they were assigned runway 27R. However, they preferred a longer runway due to their landing weight and were therefore assigned runway 28C which they had to load into the flight management computer and re-brief. The captain indicated that the seat belt sign had been turned on early in the descent during the “arrival” passenger announcement. About 13,000 ft, the captain gave the cabin crew the “double chime” indicating that they should prepare the cabin for landing and then take their seats. The flight crewmembers reported that as the aircraft was descending through an isolated overcast layer, they observed a small cloud buildup with the top of the buildup slightly above their altitude. They contacted ATC and requested and were approved to make a left turn to avoid the cloud build-up. Although they attempted to avoid the cloud, the aircraft penetrated the left outermost area of the cloud buildup at an altitude of approximately 12,100 ft. The flight crewmembers indicated that as they went through this area, they experienced about 5 seconds of light to borderline moderate turbulence. As soon as they exited the area, the air was once again smooth. Shortly after the encounter, the flight crewmembers received a call from the cabin crew informing them that one of the flight attendants had fallen in the rear galley and had injured her foot. According to the cabin crew, shortly after they heard the double chime, there was a big bump of turbulence, and a flight attendant lost her balance, twisted her ankle, and hit her back on the corner of her jump seat. A physician who was onboard the airplane assisted her and helped move her to a passenger seat for the remainder of the flight. Emergency medical personnel met the airplane at the gate and a post-flight medical evaluation revealed that the flight attendant had a fracture injury to her metatarsal bone in the left foot. Based on a review of WSR-88Dweather radar (KMKX), satellite (GOES-16), and upper air model (HRRR) data, the turbulence encounter appeared generally coincident in time and location with a convective updraft that reached heights above 12,100 ft.