MINNEAPOLIS - Dr. Mindy Rauch thought she was humoring her brother at a family gathering last Father's Day when she agreed to examine his 6-month-old daughter, Hannah. The baby's stomach worried him: it felt taut, and too big. Dr. Rauch, a pediatrician, thought her brother was just another nervous first-time father. "Oh, look at this belly," she said playfully, feeling the baby's middle. A moment later she turned to her husband, who is also a pediatrician, and said, "You have to feel this." Hannah's liver and spleen were enormously enlarged. It was a sign of serious illness: possibly a tumor,...