David L. Robinson, 16, the McLean High School football player stricken Monday morning during the team's first workout of the season, died of an enlarged heart, Northern Virginia's medical examiner said yesterday.

The condition, in which death can be triggered suddenly by exercise or other stresses that tax the heart, often occurs at birth, physicians said yesterday, but medical examiner James C. Beyer said that it may never be known how Robinson developed the fatal condition.

Physicians said that enlarged hearts generally result in early death, and that it was unlikely that Robinson's collapse was due to an excessive workout at the school. The condition is difficult to detect in routine physicals and, even if Robinson's condition had been detected, "There's little that can be done about it," said Mohamed K. Mardini, a pediatric cardiologist at Fairfax Hospital.

Robinson, of the 2800 block of Hollywood Road near Falls Church, was working out lightly with other teammates about 10:15 a.m. Monday when he collapsed, school officials said.

Two trainers administered cardiopulmonary resuscitation before Robinson was flown by helicopter to Fairfax Hospital. Doctors worked on him more than an hour before he was pronounced dead at 11:41 a.m.

The medical examiner's office will conduct toxicological tests, which Beyer said are routine. The results are expected in two to three weeks.

Robinson, who was 6 feet 2 and about 270 pounds, was a tackle. He played frequently last year and was expected to be one of the team's starters this year.

The school's coach, Karl Buckwalter, who was overseeing his first football practice at McLean on Monday, could not be reached for comment, and Robinson's parents declined to comment.

Officials at McLean High School barred reporters from the school's practice field yesterday, and declined to comment on the autopsy report. All calls were referred to the Fairfax County Public Schools central administrative office.

"We follow the regulations of the people who know sports and know medicine," said Dolores Bohen, a school spokeswoman. "For us it was really important that we know any youngster who participates in sports in Fairfax have a required physical examination."

Robinson had passed a physical Saturday at the Emergency USA clinic in Fairfax, not at Group Health as earlier reported. However, physicians yesterday said that only sophisticated and expensive tests can detect enlarged hearts.

It could not be learned yesterday whether Robinson knew of his heart problem. A relative said no one in the Robinson family had a history of heart problems.

Doctors suggested that enlarged hearts usually lead to death sooner or later. "It would have happened to him doing something else," Mardini said.

Death among people who have enlarged hearts generally is triggered by heat or exercise. Exercise in itself would not cause the heart to thicken to a dangerous degree, Mardini said.

An enlarged heart is detected by ultrasound scan during a test that costs about $300, said Dennis Rivenburgh, a certified athletic trainer and administrator of the Shady Grove Center for Sports Medicine. Unless an irregular heartbeat is detected using traditional means, an ultrasound test is not called for, Rivenburgh and other doctors said.

A person with an enlarged heart often has a normal heartbeat, doctors said.

Physicians yesterday said that death from an enlarged heart is no more common for athletes than for other people. However, athletes' deaths often get more attention because of their prominence.

Robinson was the first Washington-area high school student to die after school-sponsored athletic activity since Linthicum, Md., senior Jonathan Ausby III, 17, who died of an enlarged heart three years ago during a basketball game.