From PEOPLE Magazine Click to enlarge
As Hank Lawson, the quick-thinking, house-calling Hamptons doctor he plays on USA's hit series Royal Pains, Mark Feuerstein never met a medical mystery he couldn't solve. As a father in real life, he has already grappled with a health crisis more heart-wrenching than any scripted drama. When the actor's third child, Adelaide, started behaving strangely in November 2009, at only 6 weeks old, Feuerstein was at first unfazed. "She'd cry a lot," recalls his wife, Dana Klein, "especially when she ate. Then she wasn't gaining weight." Feuerstein chalked it up to colic, and five months later, when a gastroenterologist prescribed her Pepcid, "it was having a bit of an effect," he says. "I was going, 'It's just reflux, honey.'"

But it wasn't. At six months, Addie was diagnosed with ALCAPA, a rare and potentially fatal heart defect (see box). "I've never been more aware of how little I know playing a doctor," says Feuerstein, 39, "than I did learning about this." When Klein, 36, took Addie to a cardiologist last March, the baby was rushed immediately to Children's Hospital Los Angeles, where she stayed for three months. "To see your child sweating, yet lying still, because her heart is working overtime, you can imagine the feeling of helplessness," says Feuerstein with a shudder.

After an initial operation confirmed the defect, Addie was kept in the ICU to monitor whether her heart muscle would recover. Klein visited her daily, while Feuerstein, shooting Royal Pains in New York, flew home on weekends. Family and friends sent over food; their older kids Lila, 4, and Frisco, 2, prayed and donated blood and platelets for Addie's surgeries. "When you get past the horror of what's going on," says Feuerstein, "you see all the people coming together for us."

By mid-May, "Addie had been on medications for so long, she was just delirious," Klein says. "She cried for 12 hours straight." The next day, surgeon Vaughn Starnes went in to repair Addie's mitral valve. Right before, "I remember climbing onto the gurney," Klein says, "and saying to her, 'You're so strong. You can do this.'" The couple, meanwhile, found solace in their own strong bond. "You absorb as much of the other's energy as you can," Feuerstein says, "because there's no place for ego anymore."

The surgery was a success, and Addie was finally able to go home in June. Now 16 months old, she's "back on the growth chart," says Klein. Adds Feuerstein: "She fought the whole way. What obstacle in life will be hard for her? 'I failed a pop quiz? Too bad! I had a heart defect!' Nothing can touch her now."