Final answer:
The nurse should first auscultate the client's lungs to assess for any pulmonary issues that might be causing the weakness in the legs.
Step-by-step explanation:
The nurse should first auscultate the client's lungs. Furosemide is a diuretic medication commonly used to treat conditions like edema and hypertension. One potential side effect of furosemide is hypokalemia, which is a low level of potassium in the blood. Hypokalemia can cause muscle weakness and fatigue.
Auscultating the client's lungs can help determine if the weakness in the legs is due to a pulmonary issue, such as fluid accumulation or difficulty breathing, which may be related to the medication. This would be the most immediate concern and would help guide further assessment and intervention.