In a contentious confirmation hearing to become the nation's top health official, Robert F. Kennedy Jr. struggled to answer questions about Medicare and Medicaid, programs that affect tens of millions of Americans,

Robert F. Kennedy Jr., nominee for HHS secretary, seemed unfamiliar with the massive insurance program during Thursday’s hearing, mistakenly saying Medicare Part A mainly paid for primary care or physicians.

While Mr. Kennedy, seeking the job of health secretary, has been vocal about vaccines and his desire to overhaul the nation’s diet, he has said very little about other issues.