Artistic swimmer Anita Alvarez "is healthy" but did not compete in Friday's team event after fainting in the pool at the World Aquatics Championships in Budapest on Wednesday.
American team doctor Selina Shah said she was "very confident" Alvarez could have competed on Friday.
But international federation Fina said "she should not compete".
Alvarez previously fainted following a routine during an Olympic qualifier in Barcelona last year, with coach Andrea Fuentes having also rescued her on that occasion.
Dr Shah told AFP stopping Alvarez from competing on Friday "was a decision Fina had made.".
"In my opinion she could have competed, I am very confident," continued Shah.
"I provided all medical evidence from Anita. I'm not aware of their decision-making process."
A Fina statement said: "The health and safety of athletes must always come first. While Fina understands why this decision will have been disappointing to the athlete, it was a decision that was made with her best interests in mind."
Dr Shah added that the team did not yet know what caused Alvarez to faint.
"We are going to look into what happened and do some additional extensive programmes with consultants and make another determination as to an actual cause," she said.
USA Artistic Swimming said: "Anita is healthy and has been evaluated extensively by a medical team. She is doing well and thankful to everyone for their support."
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