WASHINGTON, D.C — President Donald Trump on Monday used the platform of the presidency to promote unproven and in some cases discredited ties between Tylenol, vaccines and autism as his administration announced a wide-ranging effort to study the causes of the complex brain disorder. He also fueled long-debunked claims that ingredients in vaccines or timing shots close together could contribute to rising rates of autism in the U.S., without providing any medical evidence. One challenge is that it’s hard to disentangle the effects of Tylenol use from the effects of high fevers during pregnancy. Trump also urged not giving Tylenol to young children, but scientists say that research indicates autism develops in the fetal brain. Shares of Kenvue Inc. fell 7.5% in trading Monday, reducing the company’s market value by about $2.6 billion.