The young athlete, now competing at the Rio Games, always considered herself to be a girl just like the others, a girl who loved to run. Then the governing body of track and field told her she was different, so different that her track career could be over.
Marked "confidential" and signed "best sporting regards," the letter outlined a choice for the athlete: Open herself up to a panel of medical experts who could recommend surgery or chemical treatment to reduce her testosterone levels, or stop competing.
She had fallen foul of the International Association of Athletics Federations' rules aimed at providing a fair playing field for women by keeping out athletes with high testosterone, a naturally occurring strength-building hormone.
The IAAF's medical director at the time told track authorities in the athlete's country that blood and urine tests detected testosterone levels that were "abnormally high." The suspected cause, wrote Dr. Gabriel Dolle, wasn't doping but another hot-button issue likely to flare in the Rio Olympics' final week: hyperandrogenism.
Had the athlete not been a runner, she might never have known of her condition. It was flagged by IAAF tests that look for banned drugs. The athlete was stunned when told that her testosterone pointed to hyperandrogenism, her then-coach told The Associated Press.
"She couldn't understand. It was shock," the coach said. "I said, 'You're not alone. There are others.'"
Thus started a months-long process of trips to foreign clinics for batteries of tests and potentially life-changing choices.
The AP will not name the athlete, her country or give details that could identify her. In message exchanges, the athlete told AP she is focusing on competition in Rio de Janeiro and that her story is "personal and private."
However, the IAAF letter and AP's interview with the coach, who was intimately involved in her eventual decision to agree to testosterone-curbing treatment, shed unprecedented light on the inner workings of the process that at least 14 women have gone through since the federation introduced it in 2011.
Another athlete has become the unwilling face of this complex and sensitive issue. Caster Semenya will race and likely win gold in the 800 meters. Believed to be hyperandrogenic, outed as physiologically different without her consent when she won the world title in 2009, the South African's dominance has again pushed to the fore divisive questions about whether allowing women to compete with testosterone far above the female norm is fair and whether the hormone's attributed performance-enhancing effects are significantly greater than other natural gifts, like height for basketball players.
Having not withstood a legal challenge brought by another female athlete, India's Dutee Chand, the IAAF regulations are now on hold, suspended by the Court of Arbitration for Sport since July 2015. That means hyperandrogenic women can compete in Rio without reining in their natural testosterone levels.
During the IAAF process, the unidentified athlete could not compete. The coach covered her absence from races by lying that she was injured. The AP will not identify the coach to avoid identifying the athlete.
The first IAAF-requested tests to determine her exact condition required a trip to another nation's capital, where she was met by Dolle, and that was followed by two trips to a clinic in Nice, France, the coach said.
The first Nice trip was accompanied but the second, lasting over a week, was not because the IAAF did not want to pay for someone to go with her, said the coach.
"I didn't want her to go alone. She didn't speak French very well. I was afraid she wouldn't understand," the coach said. "I said, 'Before you take any medicine, call me. Don't take anything.' The doctors say, 'It's for her good.' And I thought to myself, 'She's fine as she is.'"
After the second Nice trip, subsequent correspondence from Dolle offered two choices: surgery or medicinal treatment, said the coach. The coach urged the athlete not to go under the surgeon's knife, saying: "We can't take this risk; cut things off that God gave you ... When things are cut off, it's forever. You can't get those parts back."
"She said, 'Since I love this sport, I'll take the medicine,'" the coach said.
The treatment caused weight-gain and the unpleasant side-effect of making the athlete smell "awful," sweaty and dirty, even though she stayed clean and wore deodorant and perfume, the coach said.
When the IAAF eventually allowed the athlete to resume competing, she was delighted. But CAS only suspended the regulations, rather than overturn them entirely, giving the IAAF until July 2017 to produce evidence that high testosterone gives hyperandrogenic women a significant performance advantage.
"She is free but she is scared that from one day to the next they could stop her from running again," the coach said. "She's in limbo, waiting for something bad to happen. She asks, 'Are they going to stop me again? Are they going to make me take more medicine?"