In the summer of 2019, Karson Lewis, then 23, was hiking with friends around Belton Lake, north of Austin, Texas when her legs suddenly stopped working. "It felt like I was stuck in quicksand, like my shoes were full of lead," she recalled. She staggered over to a fence, where she steadied herself and caught her breath, blaming her symptoms on heat exhaustion. When other gait changes prompted a visit to an orthopedic surgeon in subsequent weeks, she received a diagnosis of bursitis and a steroid shot in the hip...