The Obama administration made it a priority to keep down the cost of insurance on the exchanges, the online marketplaces that are central to the Affordable Care Act. But one way that insurers have been able to offer lower rates is by creating networks that are far smaller than what most Americans are accustomed to.

The result, some argue, is a two-tiered system of health care: Many of the people who buy health plans on the exchanges have fewer hospitals and doctors to choose from than those with coverage through their employers.

A number of the nation’s top hospitals — including the Mayo Clinic in Minnesota, Cedars-Sinai in Los Angeles, and children’s hospitals in Seattle, Houston and St. Louis — are cut out of most plans sold on the exchange.

In most cases, the decision was about the cost of care…

“It just stresses me,” said Blank, 53, a self-employed massage therapist who is sorting through his options but readily admits that his family has benefited from other parts of the health law. “I hope things continue wonderfully for my daughter and that she doesn’t need the level of care she got after her diagnosis, but there’s this unknown.”