CHICAGO — Wearing a surgical gown and a mask, 9-year-old Adam Litwin watched in awe as his grandfather, a podiatrist, mended a fractured foot. “I was just mesmerized,” Litwin recalled. “I literally knew from that moment on that there was nothing else I wanted to do with my life.” He began to ask for medical posters and textbooks for his birthday. He had his own stethoscope. In his teens he wore a