The nation’s largest health-care provider greatly expanded its already extensive cancer treatment business when it bought U.S. Oncology for $2.2 billion last December. Still, distribution remains McKesson’s backbone: A full one-third of all medicines used in the U.S. run through its pipeline.The company also is the dominant player in health care information systems. More than 70% of the nation’s big market hospitals use its technology to digitize prescriptions and patient medical records. A class-action lawsuit over wholesale drug prices has cut into profits; still, its stock is up 23% over the past year. —P.N.