Final answer:
The nurse should prioritize the white blood cell level when caring for an older adult with severe Clostridium difficile-related diarrhea, as it indicates the severity of infection.
Step-by-step explanation:
When a nurse is caring for an older adult with severe Clostridium difficile-related diarrhoea, prioritizing which laboratory test to review is critical. Among the options provided, the nurse should prioritize the white blood cell level. This is because an elevated white blood cell count is indicative of an infection, which is consistent with a case of C. difficile. Additionally, Clostridium difficile infection can lead to severe colitis and systemic infection, which would likely lead to increased white blood cells in response to the infection. While the other tests are also important, the white blood cell count will provide immediate information about the severity of the infection and inform the urgency of treatment decisions.