The diagnosis of what ailed Vladimir Kara-Murza came at a delicate political moment for the United States and Russia, as President Trump had just brushed aside criticism of Russian President Vladimir V. Putin as a "killer."

"There are a lot of killers," Mr. Trump had said in the interview on Fox News on Sunday. "You think our country's so innocent?" The Kremlin then demanded that Fox News apologize for the host Bill O'Reilly's characterization of Mr. Putin.

In Moscow, Mr. Kara-Murza, 35, has been in a coma and at the center of a politically hued medical mystery since Thursday as doctors puzzled over his symptoms while keeping him alive on artificial respiration.

Mr. Kara-Murza suffered similar symptoms in 2015 and later said he had been poisoned. Then, a French laboratory found elevated levels of heavy metals in his blood but was unable to pinpoint any specific poison.