Final answer:
The nurse should instruct the client to avoid exposure to sunlight when taking doxycycline for Helicobacter pylori infection.
Step-by-step explanation:
When reinforcing teaching with a client who has a prescription for doxycycline for the treatment of Helicobacter pylori infection, the nurse should include the instruction to avoid exposure to sunlight. Doxycycline is a tetracycline antibiotic that can increase sensitivity to sunlight and cause severe sunburns or rashes. It is important for the client to protect their skin from the sun when taking this medication.