David Velasquez learned his first clinical lesson early on: The health care system wasn’t made to care for people like him. Mr. Velasquez, 24, never had a primary care physician, because his parents, immigrants from Nicaragua, couldn’t afford the bills. When he was 12, his undocumented godmother died of cancer, having avoided hospitals until it was too late. Mr. Velasquez , the only college-bound member of his family, knew he needed to become a doctor.

When he registered for the Medical College Admission Test, or MCAT, his junior year, he called the Princeton Review to ask for a discount on a $1,200 preparation package. He felt that, coming from a poor Los Angeles community without connections, he would need an especially strong test score to compete for admission.

“I thought maybe they’d hear I was desperate and put me on a payment plan,” said Mr. Velasquez, now in his third year at Harvard Medical School. The company refused his pleas for a discount, so he worked overtime to scrounge up the money. After purchasing the package, he checked his bank account to see what remained: $4.80.

American medical schools are the training grounds for a white-collar, high-income industry, but they select their students from predominantly high-income, and typically white, households. Ten years ago, a national study found that over 75 percent of medical school students came from the top 40 percent of family income in the United States, representing an annual income above $75,000. A study last year from the Association of American Medical Colleges re-examined medical school demographics and found that the numbers had barely budged. Between 1988 and 2017, more than three-quarters of American medical school students came from affluent households.