Final answer:
The most appropriate first aid response to a knee injury during a soccer game is to apply ice to the knee, avoiding attempts to straighten the leg or administer medication.
Step-by-step explanation:
During a soccer game, an 18-year-old woman injured her knee which is now in a flexed position and obviously deformed. The appropriate first aid response should be to apply ice to the knee, as this can help with swelling and pain. It is not advisable to attempt to straighten the leg or administer pain medication, as these actions should be performed by medical professionals. It's also essential to avoid elevation before assessing the severity of the injury due to the risk of causing further damage.