Final answer:
The correct instruction from the nurse is to take misoprostol with food to prevent stomach upset, as misoprostol helps protect the stomach lining from the erosive effects of NSAIDs.
Step-by-step explanation:
The nurse should teach the client that when starting to take misoprostol while on long-term NSAIDs for arthritis, it is important to take misoprostol with food to prevent stomach upset. This is crucial because misoprostol is used to protect the stomach lining from the erosive effects of NSAIDs, which can cause gastrointestinal ulcers and bleeds. NSAIDs work by inhibiting cyclooxygenase, thereby reducing the synthesis of prostaglandins that leads to inflammation and pain. However, prostaglandins also protect the gastric mucosa, so their inhibition increases the risk of gastrointestinal side effects, which misoprostol, a prostaglandin analog, helps mitigate. Therefore, the correct answer to the nurse's instruction would be: C. Take misoprostol with food to prevent stomach upset.