On December 15, 2019, about 0627 eastern standard time, Piedmont Airlines flight 4797, an Embraer EMB-145LR, N648AE, encountered turbulence while on approach to land at Philadelphia International Airport (KPHL), Philadelphia, Pennsylvania. Of the 38 passengers and 3 crew members onboard, the flight attendant was seriously injured. The airplane was not damaged. The regularly scheduled domestic passenger flight was operating under the provisions of 14 Code of Federal Regulations Part 121 from Columbia Metropolitan Airport (KCAE), Columbia, South Carolina. The captain was the pilot flying, and the first officer was monitoring. As the flight approached KPHL, the first officer made a public address announcement to the flight attendant to prepare the cabin for arrival. While descending through about 10,000 feet for the visual approach to runway 35, the crew noted a cloud layer below them and, in order to transit the cloud layer quickly, the captain increased the descent from about 1,000 feet per minute (fpm) to about 1,300 fpm. When the airplane entered the cloud layer at about 7,000 feet, it encountered moderate turbulence along with icing conditions. The captain stated that to prevent an overspeed and mitigate turbulence effects, he immediately reduced the thrust to idle and inadvertently pressed the autopilot disconnect button instead of the Touch Control Steering (TCS) button, which he had intended to activate in order to manually raise the airplane's nose. He stated he maintained control of the airplane, eased the nose up to level flight, and immediately reengaged the autopilot. The crew stated the turbulence lasted about 25 to 30 seconds. At the time of the turbulence encounter, the flight attendant (FA) was in the aisle conducting her before landing compliance checks. She was at the rear of the cabin turning around when she was thrown to the floor, injuring her ankle. Medically qualified passengers assisted the FA with the injury and another passenger manned the FA’s jumpseat during landing. Emergency medical personnel met the airplane at the gate and transported the FA to the local hospital where she was diagnosed with a fractured ankle.