Final answer:
The nurse should first check the function of the portable wound drainage system when observing excessive serosanguineous drainage on the mastectomy dressing.
Step-by-step explanation:
When a client who had a mastectomy returns from surgery with a dressing and a portable wound drainage system in place, and the nurse observes an excessive amount of serosanguineous drainage on the mastectomy dressing, the nurse's next action should be C. Checking the function of the drainage system. Excessive drainage can be a sign that the drainage system is not functioning correctly, and assessing this should be the first step. If the system is not working properly, it might be causing the build-up of fluid that should otherwise be drained away, which can lead to complications. If the drainage system is functioning and there is still an excessive amount of drainage, or if there are any additional signs of complications, the nurse should then notify the surgeon.