Final answer:
The most important information to provide in a change-of-shift report is the client's urine output, as it indicates the effectiveness of furosemide and the need for careful monitoring.
Step-by-step explanation:
When providing a change-of-shift report for a client who received 40 mg of furosemide by mouth, the most important information to relay to the next nurse would be 'The client's urine output was 1500 mL over nine hours'. This information is critical because furosemide is a diuretic that increases urine production to remove excess fluid from the body. A high urine output indicates that the medication is working, but it also requires careful monitoring for signs of dehydration or electrolyte imbalance. The nurse must be aware of this to continue monitoring the client effectively.