Which circumstance permits the use of furosemide in a racehorse?

The use of furosemide in racehorses is governed by specific regulations to ensure fair competition and the health of the horses. The correct circumstance that permits its use is that the trainer must announce its use at entry time. This requirement is in place to maintain transparency among all participants, including other trainers, owners, and the officials overseeing the race. By making this announcement, it ensures that everyone is aware that the horse will be treated with furosemide, allowing for a level playing field during the race.

This practice is particularly important because furosemide is often administered to help prevent bleeding in the lungs, a condition known as exercise-induced pulmonary hemorrhage (EIPH). Announcing its use in advance allows for better monitoring of the horse's health and treatment protocols during race day and contributes to the integrity of the sport.

The other circumstances listed do not align with the established protocols surrounding furosemide use and do not ensure the necessary transparency or competitive fairness.