Final answer:
The registered nurse should recommend ibuprofen for a client with primary dysmenorrhea as it effectively blocks pain and inflammation enzymes and has a suitable safety profile.
Step-by-step explanation:
For a client with primary dysmenorrhea seeking pain relief, the registered nurse (RN) should recommend ibuprofen as the first choice. Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) that works by blocking the enzymes involved in pain and inflammation, making it an effective option for menstrual pain. It is often preferred over other options due to its balance between efficacy and safety profile. As an OTC medication, ibuprofen is more likely to deliver targeted pain relief specific to dysmenorrhea without the side effects and risks associated with opioids or the lesser anti-inflammatory effects of acetaminophen.