Cardiothoracic (Heart) Surgeon
Dr. Nelson examines a model of the human heart. Photo from LDS.org.
Elder Russell M. Nelson, who has three doctorates, first became a full-fledged M.D. at the age of 22. In his illustrious medical career, he is most well known for working on the team that developed the first heart and lung machine, enabling surgeons to perform more complex procedures during open heart surgery.
As his LDS.org profile says, "Over the years, he literally touched the hearts of thousands of patients, including top Church and civic leaders. In 1972, he performed heart surgery on Elder Spencer W. Kimball. Following the surgery he received a personal witness that his patient would someday become President of the Church."