Roy Medvedev, left, said his twin brother, Zhores, died of a heart attack a day after their 93rd birthday. (Heritage Image Partnership Ltd./Alamy Stock Photo)

Zhores Medvedev, a scientist and one of the most prominent political dissidents in the former Soviet Union, whose writings exposed quackery and fraud in Soviet scientific programs and led to his arrest and eventual exile from his homeland, died Nov. 15 in London. He died one day after his 93rd birthday.

His death was confirmed to Radio Free Europe by a friend, writer Semyon Reznik. Dr. Medvedev’s twin brother and fellow dissident, historian Roy Medvedev, told Russian news agencies that his brother had a heart attack.

Dr. Medvedev worked at leading Soviet laboratories early in his career and published nearly 100 research papers before his political activism derailed his scientific career. With expertise in microbiology, biochemistry and genetics, he grew particularly alarmed at the ideas propagated since the 1930s by Trofim D. Lysenko, a scientific charlatan who captivated the imagination of Soviet leader Joseph Stalin.

Lysenko, who denied the existence of genes, believed that plants and animals could be magically transformed or “educated” by force of will and exposure to Soviet ideals. Among other things, he said wheat plants could be changed to rye and that seeds soaked in freezing water could adapt to cold climates. Orange trees, he predicted, would one day grow in Siberia.

When these notions were put into practice, they inevitably led to disaster: rotting crops, soil depleted of nutrients and, ultimately, widespread famine. Nevertheless, Lysenko held sway over Soviet agricultural practices for years, and his influence could still be felt until Nikita Khrushchev was ousted as the country’s leader in 1964.

By then, Dr. Medvedev had been at work for three years in writing a history of Lysenko and his harmful doctrines. He worked with other scientists, including physicist Andrei D. Sakharov — who later received the Nobel Peace Prize — to expose Lysenko as a fraud.

Dr. Medvedev’s study of Lysenko was not approved for official publication in the Soviet Union, but samizdat, or clandestine, copies circulated among the intelligentsia. In 1969, the book was translated into English and published as “The Rise and Fall of T.D. Lysenko.”

Dr. Medvedev was fired from his job at an agricultural research laboratory, and within a few months was summoned to a meeting with a psychiatrist, on the pretext of discussing the behavior of his teenage son. Instead, Dr. Medvedev was taken to a holding cell, where he managed to pick the lock and walk away.

Soon afterward, on May 29, 1970, as Dr. Medvedev recounted in his book “A Question of Madness,” he was confronted at his home by two psychiatrists accompanied by several police officers.

“ ‘If you refuse to talk to us,’ one of the psychiatrists told Dr. Medvedev, ‘then we will be obliged to draw the appropriate conclusions . . . And how do you feel yourself, Zhores Aleksandrovich?’

“I answered that I felt marvelous.

“ ‘But if you feel so marvelous, then why do you think we have turned up here today?’

“ ‘Obviously, you must answer that question yourself,’ I replied.

“A police major arrived.

“ ‘ And who on earth might you be?’ Dr. Medvedev asked. ‘I didn’t invite you here.’ ”

“He protested, to no avail, that the homes of Soviet citizens were considered private and inviolable to the forces of the state.

“ ‘Get to your feet!” the police major ordered Dr. Medvedev. ‘I order you to get to your feet!’ ”

Two lower-ranking officers, twisted Dr. Medvedev’s arms behind his back, forced him out of his house and into an ambulance. He was driven to a psychiatric hospital.

The preliminary diagnosis was “severe mental illness dangerous to the public,” and Dr. Medvedev was repeatedly warned to stop his “publicist activities.”

Meanwhile, his brother, Sakharov and other activists for greater openness in the Soviet system sent telegrams and published open letters calling for Dr. Medvedev’s release. One of his friends, the novelist Alexander Solzhenitsyn, then still living in the Soviet Union, condemned Dr. Medvedev’s detention with a bold and blistering statement.

“The incarceration of freethinking healthy people in madhouses is spiritual murder,” he said. “It is a fiendish and prolonged torture . . . These crimes will never be forgotten, and all those who take part in them will be condemned endlessly, while they live and after they’re dead.

“It is shortsighted to think that you can live constantly relying on force alone, constantly scorning the objections of conscience.”

Solzhenitsyn received the Nobel Prize for Literature later that year.

Dr. Medvedev was released after 19 days. In the meantime, he and his brother wrote an account of the ordeal, “A Question of Madness,” which was smuggled out of the Soviet Union and published in the United States in 1971.

Officers promptly went to Roy Medvedev’s apartment in Moscow and seized his papers. He was fired from his job at a research institute. Dr. Medvedev, in the meantime, was assigned to a laboratory to study gerontology researcher.

When he tried to present a paper in 1972 at a scientific conference in Kiev, Ukraine — then part of the Soviet Union — plainclothes officers seized him, fearing what he might say in public, and sent him back to Moscow.

In December 1972, Dr. Medvedev received a rare visa to travel to Great Britain, where he was scheduled to spend a year working in a medical research laboratory. He moved there with his wife and one of their two sons; the other son was detained in a Soviet jail.

While living in England, Dr. Medvedev published a book about Solzhenitsyn and his battles against Soviet authorities, which seemed to be the final straw. When he sought permission to travel to a conference in California, he went to the Soviet embassy. Instead, his passport was seized, and his citizenship was revoked.

Dr. Medvedev stayed in London, where he became a medical researcher and wrote widely on science and Soviet history. In a 1979 book, “Nuclear Disaster in the Urals,” he revealed that a 1957 nuclear explosion in the Soviet Union claimed hundreds of lives. He also revealed that a 1960 rocket explosion killed dozens of members of the Soviet space program.

In 1989, government officials finally admitted that the 1957 nuclear explosion Dr. Medvedev had described years earlier actually took place.

“I feel better they finally have recognized that all this secrecy has made them suffer,” he said, “and that it’s better people know about this.”

He also published a history of Soviet agriculture and a biography of Mikhail Gorbachev, the Soviet leader who steered the country toward closer relations with the West.

Zhores Aleksandrovich Medvedev and his brother were born Nov. 14, 1925, in Tbilisi, Georgia, in the former Soviet Union.

Their mother was a cellist, and their father was a professor of philosophy in Leningrad, where the brothers grew up. During the Stalinist purges of the 1930s, their father was arrested and was never seen again.

The brothers vowed to uphold his legacy of free thinking and inquiry. Roy Medvedev was among the first historians in the Soviet Union to cast a critical eye on Stalin and his regime. The brothers jointly wrote a biography of Stalin that was published in English in 2004.

In 1989, as the Communist Party was losing its grip, Dr. Medvedev was allowed to visit Moscow — and to see his brother — for the first time in 16 years. He later had his citizenship restored but continued to live in London.

He was married in 1951 to Margarita Busina, and they had two sons and several grandchildren. Complete information about survivors was not available.

In 1990, Dr. Medvedev wrote an account of the 1986 nuclear disaster at Chernobyl, which he considered inevitable, with the Soviet Union’s history of scientific and bureaucratic incompetence.

“In the end, I was surprised at how poorly designed the reactor actually was,” he told the New York Times in 1990. “I wanted to write this book not only to show the real scale of this particular catastrophe, but also to demolish a few more secrets and deliberate misconceptions.”