Final answer:
The nurse is most concerned about postpartum infection for the client with known cardiac disease due to the potential for serious complications.
Step-by-step explanation:
The nurse is most concerned about postpartum infection for the client with known cardiac disease. Postpartum infection, also known as puerperal sepsis, can quickly spread and lead to serious complications such as peritonitis, septicemia, and even death. Given the client's cardiac disease, she may have a compromised immune system, making her more vulnerable to infections.