Back in 1968, at the age of 22, Donald J. Trump seemed the picture of health.

He stood 6 feet 2 inches with an athletic build; had played football, tennis and squash; and was taking up golf. His medical history was unblemished, aside from a routine appendectomy when he was 10.

But after he graduated from college in the spring of 1968, making him eligible to be drafted and sent to Vietnam, he received a diagnosis that would change his path: bone spurs in his heels.

The diagnosis resulted in a coveted 1-Y medical deferment that fall, exempting him from military service as the United States was undertaking huge troop deployments to Southeast Asia, inducting about 300,000 men into the military that year.

The deferment was one of five Mr. Trump received during Vietnam. The others were for education.

His experience during the era is drawing new scrutiny after the Muslim American parents of a soldier who was killed in Iraq publicly questioned whether Mr. Trump had ever sacrificed for his country. In an emotional speech at the Democratic National Convention last week, the soldier’s father, Khizr Khan, directly addressed Mr. Trump, the Republican presidential nominee, saying, “You have sacrificed nothing and no one.”