Zhores A. Medvedev, the Soviet biologist, writer and dissident who was declared insane, confined to a mental institution and stripped of his citizenship in the 1970s after attacking a Stalinist pseudoscience, died on Thursday at his home in London, where he had lived for decades. He had turned 93 the day before. Dr. Medvedev’s twin brother, the historian Roy Medvedev, told the Russian news media that the cause was a heart attack. He said his family had celebrated Dr. Medvedev’s birthday on Wednesday. “He had never complained about his heart,” he told RBC, a Russian news agency. “Three ambulances came,...