As a child, Katherine Aull ran around parks and yards with a butterfly net in one hand and a magnifying glass in the other.

They were the tools, she said, needed "to understand what was going on in the world."

At 18, Aull's preferred instruments now are microscopes and computers, but her passion for biology -- and for understanding the world -- has remained intact. Last month, Aull and three teammates represented the United States at the International Biology Olympiad competition in Australia and came home with a record four gold medals.

No sooner had she returned from that victory than Aull, who is called Kay by most who know her, headed overseas again. This time she was off to London, where she participated in the International Youth Science Forum for two weeks, a prize for winning the National Science Bowl.

"It's a lot of fun," the Fairfax resident said of her recent travels. "You get to meet with future colleagues from all around the world. You can push yourself, and you get to see where you stand in regards to the rest of the world."

Aull said she can't picture a summer -- even the one after her senior year of high school -- spent any other way. To qualify for the Biology Olympiad, Aull and 6,000 other high school students across the United States took a multiple-choice test. Those who scored in the top 10 percent moved into a semifinal round and endured another test to further winnow the pool to 20.

In early June, the 20 Americans gathered for an intense, two-week program at George Mason University that culminated in two more days of exams. The top four scorers went on to Australia.

For Aull, it was a familiar, albeit still grueling, process. Last year, she was among the four biology wizards chosen to compete in the olympiad held in Belarus. The team won a silver medal. Aull vowed to return for better.

This marked the second year the United States took part in the Biology Olympiad. The U.S. contests are sponsored by the McLean-based Center for Excellence in Education. In nations such as China, as many as 2 million youths turn out to take the initial qualifying exam.

Test questions in Australia ranged from identifying the anatomy of ants to determining the genetics of an enzyme. Unlike the test-driven qualifying rounds, the Biology Olympiad includes hours of practical lab work and applications of concepts to the real world. Aull recalled being handed some blood smears and asked to identify the leukocytes, or germ-fighting cells.

"It's deeper than what would be covered in high school," said Aull, who graduated in June from Thomas Jefferson High School for Science and Technology and plans to attend the Massachusetts Institute of Technology in the fall.

Her award-winning teammates were Bradford Hargreaves of Shreveport, La.; Clinton Hansen of Oneida, N.Y.; and ZeNan Chang of Santa Monica, Calif.

Katherine Aull of Fairfax recently won a gold medal at the International Biology Olympiad.