Final answer:
Postoperative exercises after a modified radical mastectomy are crucial to prevent the development of lymphedema by maintaining normal lymph flow and avoiding fluid accumulation.
Step-by-step explanation:
The nurse's instruction for a patient who recently had a modified radical mastectomy to exercise her arm postoperatively is important primarily to prevent lymphedema. Lymphedema is a condition characterized by swelling that occurs when lymph fluid accumulates in the interstitial tissue, which can happen after lymph node removal or damage. Exercising the arm can help maintain normal lymph flow, which reduces the risk of this accumulation. Exercises are aimed at gradually increasing mobility and strength, but without overstraining the affected area.