Final answer:
The most important instruction is to (C) use a second form of birth control while taking amoxicillin, as antibiotics can reduce the effectiveness of birth control pills.
Step-by-step explanation:
The highest priority instruction for a nurse to give a young woman being treated with amoxicillin (Amoxil) for a urinary tract infection (UTI) is to use a second form of birth control while on the drug. Antibiotics like amoxicillin may reduce the effectiveness of hormonal contraceptives like the pill.
Therefore, it is critical that patients are advised to use backup contraception to prevent unintended pregnancy. Irregular heartbeat, blood in the urine, and increased menstrual bleeding are not usual side effects associated with amoxicillin, and while they can occur, they are not the top priority in the context of contraceptive use and antibiotic interaction.