Federal health advisers on Tuesday recommended approval for an experimental combination drug to treat cystic fibrosis , despite unresolved questions about the benefit of one of the pill’s two ingredients. Vertex Pharmaceuticals has asked the Food and Drug Administration to approve its twice-a-day drug for patients with the most common form of cystic fibrosis , a deadly inherited disease that causes sticky mucus buildup in the lungs and other organs, leading to infections and early death.