Matthew Alkaitis, a third-year student at Harvard Medical School, is calm, friendly, and a good listener - the kind of qualities you’d want in a doctor. But though he spends 14 hours a day studying for his board exams, the 29-year-old isn’t sure how long he’ll be wearing a white coat. In September, Alkaitis, who also has a Ph.D. in biomedical sciences, will be starting a two-year fellowship at McKinsey & Co., where he’ll be advising clients in the health-care field.