Final answer:
The nurse's best response is that antiviral drug therapy should be started within 2 days of symptom onset, not 4 days.
Step-by-step explanation:
The nurse's best response to the young adult's request for a prescription for the new flu drug is option C: "Drug therapy should be started within 2 days of symptom onset, not 4 days."
Antiviral drugs, such as oseltamivir (Tamiflu), can be used to reduce the duration of flu symptoms when taken within 2 days of symptom onset. However, their effectiveness decreases significantly after 2 days.
Starting antiviral treatment after 4 days may not provide the desired reduction in symptoms. Therefore, it is important for the young adult to seek medical attention and start antiviral treatment as soon as possible.