New world 800m champion Caster Semenya has been asked to take a gender test, according to athletics' governing body.
The International Association of Athletics Federations says it demanded the test three weeks ago amid fears she should not be able to run as a woman.
IAAF spokesman Nick Davies said the "extremely complex, difficult" test results were not due for several weeks.
The South African athletics federation insists it is "completely sure" that Semenya, 18, is a female.
"We would not have entered her in the female competition if we had any doubts," said a statement.
Semenya won gold in impressive fashion on Wednesday, leaving her rivals trailing as she won in a time of one minute, 55.45 seconds.
Defending champion Janeth Jepkosgei was second, a massive 2.45 seconds adrift, with Britain's Jennifer Meadows taking bronze.
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