Final answer:
The nurse should instruct the assistant to wear gloves when handling a client's urine, to ensure safety and prevent potential exposure to infectious diseases during the measurement of urine output.
Step-by-step explanation:
When a nurse is delegating the collection of urinary output to an assistant, the most important instruction to provide is to wear gloves when handling a client's urine. This is to maintain universal precautions and ensure the safety of healthcare personnel by preventing potential exposure to infectious agents. While comparing the amount of output with intake is important for the nurse to calculate fluid balance, this task is typically not delegated to an assistant. Instructing the client to wash the urethra before voiding is not related to the task of measuring urine output. Using a clean measuring cup for each voiding is ideal to prevent cross-contamination; however, the critical safety precaution for the assistant is the use of gloves.