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The 93-year-old widow of a Wall Street financier has donated $1 billion to a Bronx medical school, the Albert Einstein College of Medicine, with instructions that the gift be used to cover tuition for all students going forward.

The donor, Ruth Gottesman, is a former professor at Einstein, where she studied learning disabilities, developed a screening test and ran literacy programs. It is one of the largest charitable donations to an educational institution in the United States and most likely the largest to a medical school.

The fortune came from her late husband, David Gottesman, known as Sandy, who was a protégé of Warren Buffett and had made an early investment in Berkshire Hathaway, the conglomerate Mr. Buffett built.

The donation is notable not only for its staggering size, but also because it is going to a medical institution in the Bronx, the city’s poorest borough. The Bronx has a high rate of premature deaths and ranks as the unhealthiest county in New York. Over the past generation, a number of billionaires have given hundreds of millions of dollars to better-known medical schools and hospitals in Manhattan, the city’s wealthiest borough.

Dr. Gottesman said her donation would enable new doctors to begin their careers without medical school debt, which often exceeds $200,000. She also hoped it would broaden the student body to include people who could not otherwise afford to go to medical school.

While her husband ran an investment firm, First Manhattan, Dr. Gottesman had a long career at Einstein, a well-regarded medical school, starting in 1968, when she took a job as director of psychoeducational services. She has long been on Einstein’s board of trustees and is currently the chair.