5:30 a.m., Brianna Pelletier gets ready for her daily pounding. As she lies on the couch, her dad beats her chest, then her back, coaxing the mucus out of her lungs. The pounding doesn't take care of everything. Brianna's held out for a long time, but a body with cystic fibrosis doesn't last forever. It doesn't matter that Brianna has a brilliant mathematical mind or that she's a shoo-in for MIT. Or even that her two best friends are beautiful, popular,...